My experience with later model Hunters is that they have quite deep cockpit
lockers. There should be plenty of space at the bottom to store some water
. But without knowing your boat very well I cant be certain. But that is the place I would consider first if possible.
Hmm.. You could also add a bladder under one of the aft berths.
Either way, its highly unlikey that 15gal is going to de-stabilize you too much.