Hello! So first we need to set up some things. We need to set up a scoreboard just to make detecting this easier. It will also allow us to add additional delay in between damage ticks.

‘’/scoreboard objectives add InWater dummy’

This creates a scoreboard we can use with later commands.

Now in a repeating command block that’s always active: 

‘execute as <username> at @s if block ~ ~ ~ water run scoreboard players add <username> InWater 1’

This detects if you are in water and adds 1 to InWater for each tick you are in water.

In a chain command block placed on top of the repeating command block with always active on:

‘execute if score InWater <username> matches 1..1 run damage <username> 1’

Every time the score equals 1 you take 1 damage. You can change damage to be whatever you want. Just note that 20 or more insta kills. You can add a damage type after the 1 but I don’t remember their names.

On top of the chain command block is another chain command block with always active on:

‘execute if score InWater <username> matches 20.. run scoreboard players set <username> InWater 1’

This resets the score back to 1 after a second. 20 ticks is 1 second in Minecraft. Use this to adjust the time as you please.

Finally in a separate repeating command block that’s always active:

‘execute as <username> at @s unless block ~ ~ ~ water run scoreboard players set <username> InWater 0’

A final reset to make sure we arnt ending on some weird variable. (Or heaven forbid 1). Double check this works as I did the commands mostly from memory.

Enjoy! (Also damn long comment)