Drowning (sometimes called "asphyxiation") is an effect found in some areas where the player is subject to regular, increasing, damage over time. Sooner or later the player will be forced to leave the area or be killed.
This damage can be totally/partially reduced by flat Defense stat and completely prevented by using an Air Bubble "potion".
Reentering the area usually resets the damage counter, but sometimes it's not possible without some kind of cost (like actively play to switch areas in Mystic Path, play Defend Mission again to get to Secret Lab, expending the Endless Dungeon entrance ticket, etc)
List of areas with this effect: