Alaska, Alaska, Alaska. I don't know your son, obviously, but to me the options in Alaska while in port are much more interesting than the Bahamas - no matter the age. And when you are on the ship, there really is no difference except it will be a little colder to swim. Our son never goes near the pools anyway, as he can't stand how they are just wall to wall kids on the warm weather cruises.
I realize all kids are different, but our son's favorite vacations have always been the outdoor adventure type - Alaska, Yellowstone, Colorado, Iceland, etc. He's a water lover as well, but can get that on other vacations or at our local pool.