Utterly Mad

I like going underwater with the power armor. I noticed that you dont need to go up for air with the power armor, BUT also noticed that when the suit gets low on power, it stops supplying oxygen and you start to drown, even after the fusion core is replaced and/or you go up for air. Can anybody explain exactly how the underwater power armor mechanics work?

Speaking of underwater power armor, there's a bit at the back of the flooded quarry where you can suddenly move at full speed and fire your gun, but you're still underwater. It looked like an area where a store or some NPC might hang out. Was a "cave" set halfway down the far wall of the quarry.