I did a kayak/snorkel trip with VI Eco Tours this past weekend and they did an excellent job! If you are looking for something to do while on St. Thomas this is definitely something you should check out. It's kid friendly as well as enjoyable for adults. The guide was very entertaining while he educates you on the importance of the mangroves and points out all the inhabitants.
The tour takes you on a kayak trip through the mangroves for about a mile and a half. Then you come to a protected lagoon that is bordered by a reef that faces the southern most tip of St. Thomas, Long Point. You disembark from the kayaks and start off on your snorkel. The best places were along the boarders of the mangroves where the fish hang for cover. The guide snorkels along side pointing out sea life that I would have missed had he not pointed it out. ie. upside down jelly fish on the bottom. After the snorkel, you get back in the kayaks and head back to the Eco Tours headquarters. The whole tour took about 2 1/2 hours which includes a 1/2 hour discussion at the beginning to overview the tour and give any instruction for newbies.
They provide plenty of water and encourage you to keep hydrated. And, if it's sunny out, definitely where sunblock and maybe a hat. Definitely bring a camera! I didn't bring mine so I don't have very many pics.