OK I have two characters with breathers (conch shells) enabled in the second weapon slot and two with UW helms. These are the ones that usually run out of air first, before the whole dang party goes belly up, so to speak

Is there some trick to operating the underwater breathers? They don't seem to make any difference to my characters.

You need to fit the breathers in the back slot, and the helmet on the head..of the same person .
You can only fit 2 persons with breathers with that equipment. Go kill some more crabs (you gets lots of shells in the coloseum). And you can fit your party fully with breathers.