__________________ "It is not so much for its beauty that the Sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
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.