Sandylynn- I have been snorkeling in St Thomas many times. Although I can't say I have ever been on any of those excursions. We normally just do it on our own. We get off the ship, grab a cab or a bus to Redhook and then a water taxi from there to St John's. Then you can go to any of their great snorkeling spots (Trunk Bay being the most popular). It's not hard to do, but I would understand if someone didn't want to risk doing it on their own.
I would recommend finding an excursion that goes to Trunk bay (never been to ship wreck cove) and or one that picks you up and drops you off from the cruise dock (this saves time in transport)
If you want more details on doing this on your own. I can help.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013