This is simply a retextured Rebreather that isn't considered a quest item, and so can be found in loot drops or purchased from vendors.
I took the base game Rebreather mesh and retextured it to look less homemade.
The water breathing effect uses consumable scrubbers that slowly get saturated with CO2 and need to be either cleaned or replaced when full. How quickly the CO2 builds up is determined by your Survival skill, so the higher your Survival, the longer each scrubber lasts. You'll be notified when the saturation reaches 50%, 75%, 85% and 95%. Once fully saturated, you won't be able to breathe underwater anymore until you either clean or replace the scrubber. Replacement scrubbers can also be found in loot drops or purchased from vendors. The rate of consumption can be adjusted by changing the
aaaGLOBResourceDrainMult global variable.