On my first cruise, stayed in a similarly situated cabin on the Aloha Deck aboard MV Sun Princess. On that cruise, I heard no noise whatsoever from the activities directly above me on the Riviera (pool) deck. Princess seems to build adequate sound insulation into their ships, so I doubt that you will have a problem with noise.
Although the cabins on the Caribe Deck do have slightly larger balconies than the cabins on the Aloha Deck and the Baja Deck aboard MV Grand Princess, it's not enough larger to offset the fact that the cabin is on a lower deck. Thus, a swap to the cabins on the Caribe Deck actually would be a downgrade rather than an upgrade.
I guess you have aready read my comments about the Champagne Catamaran Sailaway to St. John. Yes, I'm ready to fork over another $96 to do it again on my next cruise that calls at St. Thomas -- and I'm a cheap buzzard that normally books an inside cabin to cut costs.
Princess does throw a fantastic barbecue and beach party on Princess Cays, too.