Oxygen is the hard limit on every dive. Subnautica 2 offers a tiered progression of tanks: the Standard Air Tank, High Capacity Air Tank and Ultra High Capacity Air Tank, alongside the Rebreather (improves deep-depth oxygen efficiency), Scuba Mask, and Basic / Improved Fins.
Tank tier is effectively a soft depth gate: the Abyssal Trench and the Ancient Turbine descent are simply not survivable on a Standard Tank without staged Portable Oxygen Generators or forward bases. Upgrading tanks is one of the clearest “you can now go deeper” progression markers.