Try Blue Island Divers or Admiralty Dive in St. Thomas. We dove with Blue Island on our last Celebrity cruise and they pick you up at the cruise ship dock, right next to where the cruise ship "cattle boat" does. Admiralty picks you up in the same place.
After our Celebrity cruise last December, we decided that big ship cruising and diving don't mesh well for us. We were fortunate enough to have (the only) balcony on-board the ship, which we found was indespensible for our dive gear. If you don't have a balcony, you're going to have a real hard time drying your gear due to the size of the rooms and bathrooms.
Although I agree that renting a BC and possibly wetsuit is easier for the wet gear thing, I have to say that the condition of some of the BCs we saw as rentals in the Caribbean would have made me nervous.
We quickly decided that if we want to cruise, we're going to cruise. If we want to dive, we'll either stay at a resort or book a liveaboard cruise. Hence, our booking on the Nekton Pilot in Belize this December!