SCUBA gear is an item originally found in Unreal. It let's you breathe underwater until the oxygen supply ends, which is a relatively long time since it is compressed. It is activated automatically once underwater. If deactivated while underwater, you'll start drowning. It isn't a very common item, and it is mostly found in maps like Chizra - Nali Water God and The Ceremonial Chambers.
Other Appearances
The SCUBA gear appears one time in Return to Na Pali, but in an alternate version known as the Marine SCUBA Gear. What differs from the original version is the regeneration of the oxygen supply, giving it infinite usability. Also, some messages will appear that warn you about the quantity of oxygen left or when the oxygen level is returned to full.
Another appearance is in UT in the map AS-OceanFloor. You'll find four of them in the Attackers' base; they act in the same way as the normal SCUBA Gears found in Unreal, except since the "Activate Item" command is removed from the game, the Scuba Gear actuvates automatically, once you collect it (A sound of strong breath reverbrating in a mask, heard in Unreal I when underwater wearing the Scuba Gear, will validate that).