Hi brwab, not sure about the quad excursion. However, the answer to snorkeling on your own is a definite yes. You can even rent snorkeling gear at Trunk Bay (St. John) or if you choose to stay on St. Thomas, Coki Beach. It's just my opinion that snorkeling at Coki is better than Megan's. Sapphire Beach is the best, but I didn't see any rental equipment there. Trunk Bay by far is the best and it's very easy to get to. There is a complete instructional guide in the FAQs at the end of the front page, but quickly. You can catch a ferry at Red Hook (or Charolette-rates vary Red Hook is $6/pp) take a cab to Trunk Bay, pay the entrace fee ($2/pp I think, not expensive). Then enjoy one of the top 10 beaches in the world. Here
is the ferry schedule.
Best wishes and have a marvelous cruise!