The Breath Bar is a blue bar that measures the amount of air/oxygen left while under water. It is seen the health bar, and is only seen while the player is under water. It disappears once the player swims up to the surface of the water.
Immediately after running out of breath (blue bar shrinks to 0%), the player will start to drown and take damage as normal on the health bar. The player must swim up to the surface to regain their breath bar and stop taking damage.