At the "Player Entities"
Health and Max Breath are already implemented, right? 
And at "Non-Player Entities"
Mobs already drop items, dont they? 
and the Variety of entities that are either hostile or docile towards the player. .
And have a breath-meter element that appears when the player goes into an area in which they cannot breathe, is kinda implemented 
Dont know if someone mentioned this already, but here you go