Cleon
Legend
I think it's more of a "game mechanics first" design at play here. They wanted a critter that would serve as a waterskin, plain and simple.
Making up explanations for some of the monsters' weird traits is part of the fun of D&D.

If you want to develop some of the rest into flavor text, I'm not opposed.
I'd leave it as "speculation among the learned" rather than giving a straight-out answer.
Since great glacier fish "All reproduce by laying thousands of tiny eggs" I doubt it's some kind of freshwater pouch-brooding, and it's a very inefficient means of buoyancy compared to a regular swim bladder.
Maybe they actually were engineered to be living waterskins?