In the bio lab, there is a hidden oxygen tank that allows you to stay underwater longer without needing to surface for air.

Dive into the water to the left of the lift for the Pack-a-Punch, and you’ll see a stone that is slightly brighter than the others. Shoot this stone with a weapon that has explosive ammo. The stone will then float upward, releasing the oxygen tank. You can now interact with it, giving you more oxygen for underwater exploration.