So I don't know if this is a bug or one of these „features“ that empires mod has but basically if you drown and take damage, you heal back all the damage if you swim back to the surface. (all done on a server by the way) Here is a link that showcase the bug (look at the healthbar) https://plays.tv/video/5a7b3cdd8be8d9f9aa/water-dmg-bug
I like it. I'm always disappointed when games don't regenerate health after a long dive. (I was being emphatic not sarcastic)