The VI Cat is the tour operator that goes out of the dock at Frenchman's Cove. They could be ok if you want someone holding your hand snorkeling. We like the snorkeling trips by Cruz Bay Watersports, who operate The Flying Frenchman out of the Frenchman's Reef dock as well as other boats from the Ritz and Westin St John if we're looking for one directly from where we are staying. There are a bunch of other ones out there as mentioned by
@ljmiii, though you'll need a rental car or taxi for those.