If you're wanting to do a guided snorkel excursion through Disney, I can't help you.
But one of the excursions available is ST59 St. John on your own. The ferry leaves right from the ship pier, travels to St. John, allows you to have 2-3 hrs on your own on the island, and then you ferry back to the ship. Not as much to "figure out" as getting a taxi to Red Hook and taking the public ferry to/from St. John (although that is always an option).
I say this because Maho Bay in St. John is the PERFECT beach for a first time snorkeler. Both of our sons (now 15 & 12) had their first experiences there as preschoolers. I have snorkeled there many, many times and have always snorkeled with turtles and huge schools of fish. The beach is quiet and it has a lovely sand & sea grass bottom (so you won't see dramatic coral formation or colors), but it is also safe to put your feet down, if needed, without damaging any sea life. There are no waves to speak of and the entry is very gradual. Off to the sides there are rocky areas with coral if more experienced snorkelers want variety.
I know there are calm bays on St. Thomas as well, but Maho would be my choice for a first timer.