Here are my comments, for what they are worth:
San Juan, Puerto Rico - Agree with the previous writer. Best diving is either southwest or at islands far off the West Coast. Even so, hardly worth the trouble.
Bridgetown, Barbados - Some good wrecks.
Castries, St. Lucia - Good dives, but do not miss out on the natural wonders of the island.
St. John’s, Antigua - Not worth the trouble and the dive shops are scary.
Philipsburg, St. Maarten - Don’t know. We did not dive there when we went. Seemed like nothing to offer. We did go for a nice snorkel on the French side, arranged by the ship. Corals were marked with plaques stating what they were.
St. Croix, U.S. Virgin Islands - Dive the town pier. Lots of sea horses.
Finally, if you are an experienced diver, try to make your own arrangements for dives now on the Internet. Cruise dives are more expensive and they lean toward new divers. The latter may be good for you if you are relatively new. The big advantage of the cruise dives is that you are guaranteed back before sailing. If you arrange your own, tell them when you sail and get them to assure you that you will be back in time.
I’m envious!
Gerry