Recently returned from a cruise with a 4 year old and 10 month old. One of the stops was St. Thomas, so I thought I'd share our experience in case it would help someone else.
We were on Royal Caribbean, and we docked at Crown Bay. If you're doing something on your own, it's important to know where you are docked, because there are two cruise piers.
We booked a snorkeling excursion through our travel agency before arriving. The excursion left from the OTHER cruise pier, so we needed to get ourselves over there.
We did not bring any car seats or strollers with us.
Once you exit the pier, there are taxis waiting. A taxi in St. Thomas is a custom pickup truck with the bed outiftted with rows of seats. It can fit 20 people and the driver will try to jam on as many people as possible. Tell the driver where you're going and he will group passengers going to similar destinations. I think we paid $8 per person for a ride to the other cruise pier, ~15 minute ride. Kids are half off or free to ride. Just pay the driver with american dollars. The driver might make stops on the way to pick up more people. From what I understand, taxi drivers in St. Thomas are notoriously bad. So just make sure they're dropping you in the place you want to be. He didn't quite get us to our intended spot - we had to get some extra help from a security guard in a golf cart.
We went on a sailboat snorkeling excursion out to Buck Island to see sea turtles. The name of the company is Fury Charters. It's just a guy, his girl, and their sailboat. There were about 15 of us on the boat. Know that there are other options going out to Buck Island. This was definitely a small boat option. I saw some larger (and presumably cheaper) boats out there. If the seas are rough (which they were), you are going to feel it on this boat. The kids slept, so that was no big deal. It takes about 45-60 minutes to get out there. Sails up. Fun ride.
The waters at the island are calm. We saw sea turtles and fish, so that was good. We snorkeled for about 45 minutes and then headed back. We got back in probably half the amount of time. Calmer waters heading back. They provided snorkel gear, life jackets, and pool noodles. We brought our own snorkel gear for the 4 year old to make sure we had a set that would fit her.
Overall, the excursion was a fine, while not outstanding, experience. The boat was fine, the crew was fine. I think there are some things they could have done a bit better, but overall a fun experience. My biggest issue was that we had to pay full price for the kids (even the baby!). I believe we paid $130 per person. If we were to do it again, I would find one of the larger, cheaper boats to get a similar experience for much less.
Heading back to our ship, we had to wander a bit to get a taxi. I wish our snorkel host would have helped us get a taxi back - he knew we needed one. Anyway, we found one, the driver made some stops looking for more riders, and we eventually made it back. If using a taxi, give yourself plenty of time, because they will wait and look for more passengers if they're not full.
Enjoy your trip!