location file: 
GTa5\mods\update\update.rpf\common\data\ai

file name " weapons.meta " 

quoting a comment from youtube written by @HeySlickThatsMe and @TheNeoFelicity

https://youtu.be/zMZTQwcQIaQ?si=xMV4fgMvE-fbvBcI

"@TheNeoFelicity
Is the flag available in current versions? Is it as simple as adding it to a weapons.meta or is some scripting involved?

answer
@HeySlickThatsMe
Only weapons.meta edit needed, UsableUnderwater"

I serched this by find word " weaponflags " and add them " UsableUnderwater "

Sometimes it happens that the character does not move underwater, you can only rotate the camera, he doesn't change weapons. Maybe I pasted the word "UsableUnderwater" wrong somewhere.



If You can and want  please try fix this

Have fun 👍