Oxygen is a mechanic in Raft. Oxygen limits the amount of time a player can spend while underwater.
Oxygen is the amount of air a player has whilst underwater. When a player's oxygen runs out, they will take damage, and will eventually die. A player's oxygen can be refilled by returning to the surface, however it does not refill immediately. A player's amount of oxygen can be increased through the use of the Oxygen Bottle.
When drowning, the player takes 3.5 damage per second. The means the player only has slightly over 28 seconds before they die, assuming they were at full health when they started drowning.