This is a chest armor piece, like the Jetpack, that serves as a underwater respiration utility. The Scuba Tank carries Oxygen (24000 units) which can be refilled using a Gas Tank or directly from a Electrolytic Separator, which needs water to produce Oxygen.
To properly use the Scuba Tank, you have to wear a Gas Mask. This is the gear that brings the Oxygen from the tanks you're wearing on your back, up to your face in order to breathe. Without Gas Mask, you'll drown.
When equipped with the Gas Mask, the player is able to breathe underwater by changing the mode of the Scuba Tank to ON by pressing a Switch mode button assignable to keyboard in the Controls within in-game Options. Press this button twice to turn it OFF when surfaced. If not disabled, it will continue to consume Oxygen as long as the tanks are not empty.
Note : Drinking a potion while wearing the equipment will deny any of the effects the potion were supposed to give, even the beneficial potions, same happen for splash potions. If you drink the potion before putting the Gas Mask on your face and then you put it on, it will stop the effect instantly.
Scuba Tank can only be filled with Oxygen.