AC is such a snmall island, that by the second day, you will bump into the same people. Sked a tour early as you will meet people-locals and trevelers, that you can pal aroud with. Try The Winnie Estelle--I think it still sails daily.
My 2 cents: Bring your own snorkle mask as you will always have a good clean fit and a free way to kill a half hour. Rent or borrow fins.