Don't miss snorkeling in St Lucia. Anse Chastnet (I think I spelled that right), is at the marine park in St lucia and has great snorkeling. Water is very clear and the sand is an odd grey black mix from the islands volcanic nature. LOADS of fish life. The cruise lines normally do an excursion to there.
In Barbados, I went diving in Carlisle Bay, which has several good wrecks. They are only in about 25 feet (the shallowest ones) and they are snorkeling trips to there.
One of my favorite snorkeling trips of all time was aboard the catamaran "Eagle" out of St. Kitts. It was a ship's tour (Princess). We boarded right in front of the ship, sailed over toward Nevis, made a snorkeling stop and sailed the rest of the way to Nevis. Had a bbq lunch up the beach from the Nevis Four Seasons Resort and sailed back.
The snorkeling was excellent, the crew outstanding and the lunch very good. Would do it again in a heartbeat.
Everyone, Thank you for your suggestions. Have booked the Land & Sea to Soufriere in St. Lucia (stop for a swim) and the Kayaking excursion (some snorkeling involved) in St. Kitts. Spent 10 days in St. Thomas/St. John in May. I wore a tee shirt, but my daughter forgot to bring one. She suffered for 3 days. Will save the snorkeling for the St. Thomas stop.
Anse Chastenet on St. Lucia is a resort, not a marine park. The snorkeling is great but in a somewhat limited area because the water depth drops off fast - much better for scuba than snorkeling but worth the effort to get there because it is such a beautiful setting with a great beach bar/restaurant and nice black sand beach (which gets HOT in the Caribbean sun!). The only problem could be getting a cab to get there -its about a 1 - 1 1/2 hour drive and the last mile in is over the worst road you'll ever see! Also, be sure to tell the taxi guy that you don't want to stop at the trinket stand outside of town (it seemed that every cab did) and that you just wat to get to Anse Chastenet. The resort has a good website by the way so you can check the place out - we'd love to stay there for a week sometime. On Barbados, the snorkeling is very good at Folkestone Marine Park by the "park" itself is kind of a dump with no beach (volcanic rocks and dirt!) and very aggressive vendors. Hint: take the cab there, walk to the right around the point and you'll come to a beautiful resort with nice sandy beach and great snorkeling over the same reef! And of course when you get to St. Thomas, make the effort to get over to St. John for the beaches and snorkeling - ST JOHN IS PARADISE!!!
On St. Lucia, Anse Chastenet is nice. I've also stopped at several other locations on the east coast while catamaran sailing. I don't know the exact locations unfornuately, only that they are between the port and the pitons.
On Antiqua, I really enjoyed Paradise reef. It has a terrific variety of coral there.
I've not had a lot of luck on Barbados, but the best has been Folkestone Underwater Park on the western shore. Calling it a "Park" is a bit overstating it though.
I've done a land and Sea excursion before. Overall, I really enjoyed it, but I rate the Sea portion way above the land portion. The catamaran ride up the coast into the pitons is awesome. Try to get the cat ride in the direction "to" the Pitons, not away. We stopped twice for snorkeling, and sailed into a small harbor used in the Dr. Doolittle movie with Eddie Murphy.
The land portion was ok. The drive in volcano is really just a few bubbling mud/sulfur pits. Interesting if you've never seen anything similar. The bus ride was fair. The highlite was the lunch they provided at a gristmill I believe. It was excellent. I'm really not into food (in general, its fuel to me :-), but I really enjoyed the setting and the fried flying fish!
More focus on the Sea and less on the land would be better IMHO. I think my trip was 6 hrs or so. A bit long.