Drowning Overhaul Mod 1 7 10/1 7 2
This is a very simple mod. Basically if you run out of air bubbles you instantly die, no taking little by little damage, once you have ran out of air you die, this makes water much more dangerous and keeping your eye on those little blue bubbles all the more important.
Download link: Drowning Overhaul Mod 1.7.10/1.7.2