Minecraft – How to test for a player wearing specific armor

Since 1.13 you can do the check and do something based on it in one command. This for example does something as the closest player who wears elytra:

/execute as @p[nbt={Inventory:[{id:"minecraft:elytra",Slot:102b}]}] run <command>

Slot 102 is the chestplate slot, 103 is the head, 101 the legs and 100 the feet.

This for example does something only if a player with a Protection 4 diamond helmet exists somewhere in the world, but still in the same position that the command is executed at and as the same executor:

/execute if @p[nbt={Inventory:[{id:"minecraft:diamond_helmet",Slot:103b,tag:{Enchantments:[{id:"minecraft:protection",lvl:4s}]}}]}] run <command>

What you could do is /testfor a player with a helmet with a custom name scuba. Test for the slot number they have it in, (helmets are 103) This command should do all that:

/testfor @p[r=2] {Inventory:[{Slot:103b,tag:{display:{Name:"scuba"}}}]}

Hook it up to a clock of your choice and if the /testfor command reads true then have a comparator ready to activate a sticky piston that pushes a redstone block to activate the command

/effect @p minecraft:water_breathing

When they leave your set radius or remove there helmet the sticky piston will deactivate pulling the redstone block activating the command

/effect @p clear

This is a list of the inventory slot numbers.

The Youtuber Dragnoz also has a great video on how to do things like this.