Too fast.

stormstrider

You starve and die from dehydration far too quickly. The average body is 4 weeks for food and 4 days for water. Take in to account your in a salt water environment we can say 2 weeks for food and 2 days for water. You spend all your time looking for bloody fish.

And then being a diver. Both open water and closed its crazy how fast your o2 drops. It needs sorting.

OldHomelessGuy

Due to how game plays and not using real time everything is speed up but to help with these there are some tricks and tips

Water Problems use http://subnautica.wikia.com/wiki/Stillsuit This reduces the overall H2O loss by 75%.
Food Problems use http://subnautica.wikia.com/wiki/Thermoblade 1 slash at a fish with this will cook them
O2 problems carry x3 http://subnautica.wikia.com/wiki/High_Capacity_Tank Holding more then 1 High Capacity Tank will boost your o2 higher levels no need to switch them out just carry them in your inventory