Pressure

VERY simple mod that adds scaled damage that's dealt when you go deep underwater.
Damage is scaled by how deep you go, and the starting depth is increased based on armor value.

VERY EXPERIMENTAL, NOT INTENDED FOR AN ACTUAL SURVIVAL EXPERIENCE

(though you can use it for that if you want to, just be very careful and read the last chunk of text beforehand :3)

I personally recommend that you use Deeper Oceans or a something similar alongside this mod.
You can put this in modpacks or whatever, I don't care, just have fun.

The actual logic is based on two things:

1. If the player is submerged
2. How deep below air the player is. This means that if a player is, for example, in water with no air pockets above it at y=20, and the surface is at y=70, they will recieve damage proportional to them being 50 blocks underwater. The reason that the water works like this is so things like small offshooting water caves won't negate the damage. (also there's like a treshhold, you won't immediately start taking damage until quite a few tens of blocks underwater)

god i hope i explained this good enough