I was just on the Beach and Snorkel excursion during 2/21 cruise. You board a ferry right next to the Magic(assuming you're at WICO). The ferry takes you to the port at St. John. There you board a taxi and are taken to Trunk Bay. You then grab a spot on the gorgeous beach, grab your provided snorkel equipment and take a very short swim out to the snorkeling area.
The other one I believe we saw also at Trunk Bay. Here you enter the water from the boat. It docks away from the marked snorkeling area but near another area.
Trunk Bay is the most gorgeous beach I have ever been to and the snorkeling was great. Lots of different fish, coral and sea vegitation. We even saw a stingray.
We did this excursion because my wife and kids did not want to snorkel but play on the beach.
woody