I would set up whitelisting on the server if you haven't.
In server.properties set
white-list=true
and place each of your friend's usernames in white-list.txt to allow them access.
Don't assume no one can connect to your server just because you haven't advertised your IP.
You don't need to change the map at all. Water World is totally possible to do in survival. I actually play it in HC Survival.
Since villages only spawn their first block layer, rest is cut off due to height limit of 255. So they spawn at y=254. You can get wood from the foundations of the houses.
Also in Swamp biomes you can find witches huts, just the foundation but wood goes all the way down from y=254 to ocean bottom. Lots of wood from just 1 hut!
Also mines spawn inside solid land deep below. Wood can be found in the shafts.
In order to survive water world before 1.13 you have to spawn in an Extreme Hills or Snowy Biome so that the water at 254 is ice. You will spawn on the ice.
There are NO saplings to be found in Water World since village chests cannot spawn.
1.13 -> Even easier, wood is in shipwrecks.
Other tips:
To find seeds, after you get wood make a boat and find an ocean biome. You can tell if you are in one by seeing strange spike tips stick out of the water. It is the only piece of land that you will find in water world. Grass frequently spawns on them.
You can also find sugarcane on the spikes.
Cows can sometimes spawn at the villages, they usually will be floating in the water
Get nametags from mines and go to the nether and find a few chickens from jockeys to perm. Breed them and start a chicken farm.
Also you can start an underwater village, just convert zombie villagers and use glass or make a tube to bring sunlight down from 254 to your base. Obtain lots of stuff from trading with villagers.
Best Answer
Here is a list of where can you find chests (with loot), and what you can find in them. I included links to screenshots:
Sapling room. A normally accessible room with a bunch of dark oak saplings stacked in rows on the walls, and with a chest inside. And the chest contains... (drumroll) 25-30 dark oak saplings!
Tree chopping room. A room sealed off from the hallway with a wall, leaving a empty indent in the hall. Contains a chest with an iron axe with efficiency 1.
Wood archway room. A normally accessible room with a loot chest inside.
Curved stairway room. A room with a staircase leading from a hallway to a dead end, where is a loot chest.
Bedroom with loft. An interesting two-tiered bedroom, containing a sort of closet made of birch wood. A ladder near the entrance leads above to a loft with a loot chest.
Arena. A room with an arena/stage surrounded with dark oak fences in the center. A ladder leads up to an overhead loft with a loot chest.
Gray banner room. An altar-like room with a cobblestone construction in the middle. Behind the construction is a loot chest.
X room, Clean chest room, Fake End portal room. Secret rooms with no entrances and a loot chest.
Roof-chest room. An empty, secret room with a small attic-like space above it containing two loot chests.
All the loot chests have the same item generation rules - each chest contains 3 groups of 1–3, 1–4, and 3 item stacks. These items can spawn inside, ordered by frequency:
Rotten flesh, sting, bone, gunpowder (3-4 per chest, chance of 58% to get them from a single chest)
Wheat, bread (0.5 - 1 per chest, chance 35%)
Name tag (0.3 items per chest, chance 29%)
Coal, redstone (0.75 per chest, chance 27%)
Music disk 13, Cat, Golden apple, Diamond hoe (0.25 per chest, chance 22%)
Beetroot, pumpkin, melon seeds (0.6 per chest, chance 19%)
Iron ingot (0.5 per chest, chance 19%)
Bucket (0.2 per chest, chance 19%)
Enchanted book, Chainmail chestplate (0.15 per chest, chance 15%)
Gold ingot (0.25 per chest, chance 10%)
Diamond chestplate (0.08 per chest, chance 8%)
Enchanted golden apple (0.03 per chest, chance 3%)
The items where the number of items per chest times 100 equals the chance to obtain, will spawn in a maximum of one item per chest. In average, you will find 5 - 10 chests per Woodland Masnion.
For more informaion about rooms in woodland mansions, check out these links: A list of all rooms, or the Minecraft wiki article about Mansions.