Oxygen

Diving is the most important thing you do in Subnautica. To dive oxygen is needed to breath and you only have limited reserves.

The indicator of your oxygen status is on the left side of your screen. When your oxygen reserves are empty, the screen fades to black and after about 6 seconds you die and respawn. -30s O2 alert doesn't play until you have more than the default O2 capacity.

Also important to know is that the deeper you are, the more Oxygen is needed to breath and so the reserves of Oxygen decrease faster.

To increase the amount of Oxygen you carry with you, you can use tanks. Also you can gather the oxygen of bubbles, which are produced by Purple Brain Corals.