Minecraft – Adventure mode water bucket

The issue is that the bucket doesn't get changed to contain water, it gets completely replaced. That's just how Minecraft does it and usually it works pretty well, just not in your case.

Because player NBT cannot be directly set by commands, you can't take advantage of the full range of convenient NBT commands. Otherwise it would be a simple act of running

/data modify entity @s Inventory[{id:"minecraft:water_bucket"}].CanPlaceOn set value ["stone","dirt"]

… with a full list of all blocks in the CanPlaceOn tag. But because you cannot do that, you have two main options left:

1. Instruct the player to throw the bucket onto the ground and pick it back up. That way you can edit the bucket's NBT.
2. Replace the item in the inventory with a new bucket. This can either be pretty easy (just /clear and /give) or extremely complicated, if you don't want the bucket to move to the first slot, because then you need to do a lot of slot checking, NBT comparisons and so on.

The first option:

/data merge entity @e[type=item,nbt={Item:{id:"minecraft:water_bucket"}},limit=1] {Item:{id:"water_bucket",Count:1,tag:{CanPlaceOn:["stone"]}}}

The problem: This only makes the water placeable on stone, not all blocks. There is no way to make it placeable on all blocks without specifying all blocks. Luckily I did this long ago in this answer and have since then updated it to 1.14. So the complete command is now:

/data merge entity @e[type=item,nbt={Item:{id:"minecraft:water_bucket"}},limit=1] {Item:{id:"water_bucket",Count:1,tag:{CanPlaceOn:["acacia_button","acacia_door","acacia_fence_gate","acacia_fence","acacia_leaves","acacia_log","acacia_planks","acacia_pressure_plate","acacia_sapling","acacia_sign","acacia_slab","acacia_stairs","acacia_trapdoor","acacia_wall_sign","acacia_wood","activator_rail","allium","andesite","andesite_slab","andesite_stairs","andesite_wall","anvil","attached_melon_stem","attached_pumpkin_stem","azure_bluet","bamboo","bamboo_sapling","barrel","barrier","beacon","bedrock","beetroots","bell","birch_button","birch_door","birch_fence_gate","birch_fence","birch_leaves","birch_log","birch_planks","birch_pressure_plate","birch_sapling","birch_sign","birch_slab","birch_stairs","birch_trapdoor","birch_wall_sign","birch_wood","black_banner","black_bed","black_carpet","black_concrete_powder","black_concrete","black_glazed_terracotta","black_shulker_box","black_stained_glass","black_stained_glass_pane","black_terracotta","black_wall_banner","black_wool","blast_furnace","blue_banner","blue_bed","blue_carpet","blue_concrete_powder","blue_concrete","blue_glazed_terracotta","blue_ice","blue_orchid","blue_shulker_box","blue_stained_glass","blue_stained_glass_pane","blue_terracotta","blue_wall_banner","blue_wool","bone_block","bookshelf","brain_coral","brain_coral_block","brain_coral_fan","brain_coral_wall_fan","brewing_stand","brick_slab","brick_stairs","brick_wall","bricks","brown_banner","brown_bed","brown_carpet","brown_concrete_powder","brown_concrete","brown_glazed_terracotta","brown_mushroom_block","brown_mushroom","brown_shulker_box","brown_stained_glass","brown_stained_glass_pane","brown_terracotta","brown_wall_banner","brown_wool","bubble_column","bubble_coral","bubble_coral_block","bubble_coral_fan","bubble_coral_wall_fan","cactus","cake","campfire","carrots","cartography_table","carved_pumpkin","cauldron","chain_command_block","chest","chipped_anvil","chiseled_quartz_block","chiseled_red_sandstone","chiseled_sandstone","chiseled_stone_bricks","chorus_flower","chorus_plant","clay","coal_block","coal_ore","coarse_dirt","cobblestone","cobblestone_slab","cobblestone_stairs","cobblestone_wall","cobweb","cocoa","command_block","comparator","composter","conduit","cornflower","cracked_stone_bricks","crafting_table","creeper_head","creeper_wall_head","cut_red_sandstone","cut_red_sandstone_slab","cut_sandstone","cut_sandstone_slab","cyan_banner","cyan_bed","cyan_carpet","cyan_concrete_powder","cyan_concrete","cyan_glazed_terracotta","cyan_shulker_box","cyan_stained_glass","cyan_stained_glass_pane","cyan_terracotta","cyan_wall_banner","cyan_wool","damaged_anvil","dandelion","dark_oak_button","dark_oak_door","dark_oak_fence_gate","dark_oak_fence","dark_oak_leaves","dark_oak_log","dark_oak_planks","dark_oak_pressure_plate","dark_oak_sapling","dark_oak_sign","dark_oak_slab","dark_oak_stairs","dark_oak_trapdoor","dark_oak_wall_sign","dark_oak_wood","dark_prismarine","dark_prismarine_slab","dark_prismarine_stairs","daylight_detector","dead_brain_coral","dead_brain_coral_block","dead_brain_coral_fan","dead_brain_coral_wall_fan","dead_bubble_coral","dead_bubble_coral_block","dead_bubble_coral_fan","dead_bubble_coral_wall_fan","dead_bush","dead_fire_coral","dead_fire_coral_block","dead_fire_coral_fan","dead_fire_coral_wall_fan","dead_horn_coral","dead_horn_coral_block","dead_horn_coral_fan","dead_horn_coral_wall_fan","dead_tube_coral","dead_tube_coral_block","dead_tube_coral_fan","dead_tube_coral_wall_fan","detector_rail","diamond_block","diamond_ore","diorite","diorite_slab","diorite_stairs","diorite_wall","dirt","dispenser","dragon_egg","dragon_head","dragon_wall_head","dried_kelp_block","dropper","emerald_block","emerald_ore","enchanting_table","end_gateway","end_portal_frame","end_portal","end_rod","end_stone","end_stone_brick_slab","end_stone_brick_stairs","end_stone_brick_wall","end_stone_bricks","ender_chest","farmland","fern","fire","fire_coral","fire_coral_block","fire_coral_fan","fire_coral_wall_fan","fletching_table","flower_pot","frosted_ice","furnace","glass","glass_pane","glowstone","gold_block","gold_ore","granite","granite_slab","granite_stairs","granite_wall","grass_block","grass_path","grass","gravel","gray_banner","gray_bed","gray_carpet","gray_concrete_powder","gray_concrete","gray_glazed_terracotta","gray_shulker_box","gray_stained_glass","gray_stained_glass_pane","gray_terracotta","gray_wall_banner","gray_wool","green_banner","green_bed","green_carpet","green_concrete_powder","green_concrete","green_glazed_terracotta","green_shulker_box","green_stained_glass","green_stained_glass_pane","green_terracotta","green_wall_banner","green_wool","grindstone","hay_block","heavy_weighted_pressure_plate","hopper","horn_coral","horn_coral_block","horn_coral_fan","horn_coral_wall_fan","ice","infested_chiseled_stone_bricks","infested_cobblestone","infested_cracked_stone_bricks","infested_mossy_stone_bricks","infested_stone","infested_stone_bricks","iron_bars","iron_door","iron_block","iron_ore","iron_trapdoor","jack_o_lantern","jigsaw","jukebox","jungle_button","jungle_door","jungle_fence_gate","jungle_fence","jungle_leaves","jungle_log","jungle_planks","jungle_pressure_plate","jungle_sapling","jungle_sign","jungle_slab","jungle_stairs","jungle_trapdoor","jungle_wall_sign","jungle_wood","kelp","kelp_plant","ladder","lantern","lapis_block","lapis_ore","large_fern","lectern","lever","light_blue_banner","light_blue_bed","light_blue_carpet","light_blue_concrete_powder","light_blue_concrete","light_blue_glazed_terracotta","light_blue_shulker_box","light_blue_stained_glass","light_blue_stained_glass_pane","light_blue_terracotta","light_blue_wall_banner","light_blue_wool","light_gray_banner","light_gray_bed","light_gray_carpet","light_gray_concrete_powder","light_gray_concrete","light_gray_glazed_terracotta","light_gray_shulker_box","light_gray_stained_glass","light_gray_stained_glass_pane","light_gray_terracotta","light_gray_wall_banner","light_gray_wool","light_weighted_pressure_plate","lilac","lily_pad","lily_of_the_valley","lime_banner","lime_bed","lime_carpet","lime_concrete_powder","lime_concrete","lime_glazed_terracotta","lime_shulker_box","lime_stained_glass","lime_stained_glass_pane","lime_terracotta","lime_wall_banner","lime_wool","loom","magenta_banner","magenta_bed","magenta_carpet","magenta_concrete_powder","magenta_concrete","magenta_glazed_terracotta","magenta_shulker_box","magenta_stained_glass","magenta_stained_glass_pane","magenta_terracotta","magenta_wall_banner","magenta_wool","magma_block","melon","melon_stem","mossy_cobblestone","mossy_cobblestone_slab","mossy_cobblestone_stairs","mossy_cobblestone_wall","mossy_stone_brick_slab","mossy_stone_brick_stairs","mossy_stone_brick_wall","mossy_stone_bricks","moving_piston","mushroom_stem","mycelium","nether_brick_fence","nether_brick_slab","nether_brick_stairs","nether_brick_wall","nether_bricks","nether_portal","nether_quartz_ore","nether_wart_block","nether_wart","netherrack","note_block","oak_button","oak_door","oak_fence_gate","oak_fence","oak_leaves","oak_log","oak_planks","oak_pressure_plate","oak_sapling","oak_sign","oak_slab","oak_stairs","oak_trapdoor","oak_wall_sign","oak_wood","observer","obsidian","orange_banner","orange_bed","orange_carpet","orange_concrete_powder","orange_concrete","orange_glazed_terracotta","orange_shulker_box","orange_stained_glass","orange_stained_glass_pane","orange_terracotta","orange_tulip","orange_wall_banner","orange_wool","oxeye_daisy","packed_ice","peony","petrified_oak_slab","pink_banner","pink_bed","pink_carpet","pink_concrete_powder","pink_concrete","pink_glazed_terracotta","pink_shulker_box","pink_stained_glass","pink_stained_glass_pane","pink_terracotta","pink_tulip","pink_wall_banner","pink_wool","piston_head","piston","player_head","player_wall_head","podzol","polished_andesite","polished_andesite_slab","polished_andesite_stairs","polished_diorite","polished_diorite_slab","polished_diorite_stairs","polished_granite","polished_granite_slab","polished_granite_stairs","poppy","potatoes","potted_acacia_sapling","potted_allium","potted_azure_bluet","potted_bamboo","potted_birch_sapling","potted_blue_orchid","potted_brown_mushroom","potted_cactus","potted_cornflower","potted_dandelion","potted_dark_oak_sapling","potted_dead_bush","potted_fern","potted_jungle_sapling","potted_lily_of_the_valley","potted_oak_sapling","potted_orange_tulip","potted_oxeye_daisy","potted_pink_tulip","potted_poppy","potted_red_mushroom","potted_red_tulip","potted_spruce_sapling","potted_white_tulip","potted_wither_rose","powered_rail","prismarine","prismarine_brick_slab","prismarine_brick_stairs","prismarine_bricks","prismarine_slab","prismarine_stairs","prismarine_wall","pumpkin","pumpkin_stem","purple_banner","purple_bed","purple_carpet","purple_concrete_powder","purple_concrete","purple_glazed_terracotta","purple_shulker_box","purple_stained_glass","purple_stained_glass_pane","purple_terracotta","purple_wall_banner","purple_wool","purpur_block","purpur_pillar","purpur_slab","purpur_stairs","quartz_block","quartz_pillar","quartz_slab","quartz_stairs","rail","red_banner","red_bed","red_carpet","red_concrete_powder","red_concrete","red_glazed_terracotta","red_mushroom_block","red_mushroom","red_nether_brick_slab","red_nether_brick_stairs","red_nether_brick_wall","red_nether_bricks","red_sand","red_sandstone","red_sandstone_slab","red_sandstone_stairs","red_sandstone_wall","red_shulker_box","red_stained_glass","red_stained_glass_pane","red_terracotta","red_tulip","red_wall_banner","red_wool","redstone_block","redstone_lamp","redstone_ore","redstone_torch","redstone_wall_torch","redstone_wire","repeater","repeating_command_block","rose_bush","sand","sandstone","sandstone_slab","sandstone_stairs","sandstone_wall","scaffolding","sea_lantern","sea_pickle","seagrass","shulker_box","skeleton_skull","skeleton_wall_skull","slime_block","smithing_table","smoker","smooth_quartz","smooth_quartz_slab","smooth_quartz_stairs","smooth_red_sandstone","smooth_red_sandstone_slab","smooth_red_sandstone_stairs","smooth_sandstone","smooth_sandstone_slab","smooth_sandstone_stairs","smooth_stone","smooth_stone_slab","snow_block","snow","soul_sand","spawner","sponge","spruce_button","spruce_door","spruce_fence_gate","spruce_fence","spruce_leaves","spruce_log","spruce_planks","spruce_pressure_plate","spruce_sapling","spruce_sign","spruce_slab","spruce_stairs","spruce_trapdoor","spruce_wall_sign","spruce_wood","sticky_piston","stone","stone_brick_slab","stone_brick_stairs","stone_brick_wall","stone_bricks","stone_button","stone_pressure_plate","stone_slab","stone_stairs","stonecutter","stripped_acacia_log","stripped_acacia_wood","stripped_birch_log","stripped_birch_wood","stripped_dark_oak_log","stripped_dark_oak_wood","stripped_jungle_log","stripped_jungle_wood","stripped_oak_log","stripped_oak_wood","stripped_spruce_log","stripped_spruce_wood","structure_block","structure_void","sugar_cane","sunflower","sweet_berry_bush","tnt","tall_grass","tall_seagrass","terracotta","torch","trapped_chest","tripwire_hook","tripwire","tube_coral","tube_coral_block","tube_coral_fan","tube_coral_wall_fan","turtle_egg","vine","wall_torch","water","wet_sponge","wheat","white_banner","white_bed","white_carpet","white_concrete_powder","white_concrete","white_glazed_terracotta","white_shulker_box","white_stained_glass","white_stained_glass_pane","white_terracotta","white_tulip","white_wall_banner","white_wool","wither_rose","wither_skeleton_skull","wither_skeleton_wall_skull","yellow_banner","yellow_bed","yellow_carpet","yellow_concrete_powder","yellow_concrete","yellow_glazed_terracotta","yellow_shulker_box","yellow_stained_glass","yellow_stained_glass_pane","yellow_terracotta","yellow_wall_banner","yellow_wool","zombie_head","zombie_wall_head"]}}}

Those are 11907 characters, so there's still quite a bit of space left until the command block character limit of 32500.

The second option: For each slot, check if there is a water bucket, but not an item that can be placed, then replace if that matches:

/execute as @a[scores={buckets=1}] if data entity @s Inventory[{Slot:0b,id:"minecraft:water_bucket"}] unless data entity @s Inventory[{Slot:0b,CanPlaceOn:["stone"]}] run replaceitem entity @s hotbar.0 water_bucket{CanPlaceOn:["stone","dirt"]}

This is a shortened version of the command again, you need to list all blocks and you need one of those long commands per slot, so the full commands… don't fit into an answer. So I put them into Pastebin: https://pastebin.com/ai1RAfNp