I can help with some of the stops, first off Princess Cays, I don't think they offer any diving excursions there.
St Maarten, I went out with Trade Winds Diving a couple of years ago, nice clear water, but since we were on the Atlantic Side, lots of surge. We actually saw a whale and her calf and got into the water with them, but she took off. Good diving, but very exhausting with the surge and 5 foot swells.
Barbados, I hooked up with a company called the Wet Shop (he just recently leased his boat and gear to The Dive Shop). I ended up with the boat to myself (probably why he went out of business). The divemaster, Larraine, was the best. I have a few physical challenges and she was more than able to accomdate them and only helped me out when I was struggeling with something. She knew where all the fish and other interesting things where. Did a reef/wall dive then over to Carlisle Bay. The bay has several good wrecks in about 40 feet with great viz.
St Lucia, took my wife snorkeling at Anse Chastenet (I think that is how it is spelled). It is a resort right on the marine reserve there. Great snorkeling. Scuba St Lucia runs the dives there. Most cruise lines use them for the ship excursion. From the divers I talked to it was good diving. They did a long drift dive and a reef dive.
Never dove in St Thomas, but I understand Admiralty Dive Center is a good outfit.
A word of advice, we were there in December of 01 and my shorty was barely warm enough. I would use a full skin or full 2mm suit in Jan.