We are taking a cruise, and our focus is snorkelling. Any opinions as to our best bet for a day: we'll have about 10 a.m. to 4 p.m. Should we go to St. John or stay on St. Thomas, which is where we dock? Our cruise line has an excursion to St. John but the focus does not seem to be on snorkelling, just on beaching it. Any favored organized snorkelling tirps? (not private -- not enough money!) Thanks for any tips.