My choice I have given to 3 sets of friends who were stopping at Tortola on a cruise when we are not there. They all said it was the best day of the trip. Simon and his company do tours for several of the cruise lines but can be booked privately as well. They are familiar with cruise ship schedules and can make your plan work if doable or suggest one that does. Simon we met 15 years ago when he was the instructional skipper for TMM and remains a dear friend. He is an engaging, friendly Brit that knows every inch of the bvi above and below the water.

http://www.snorkelbvi.com/adventure.php