We have used Captain Marvin's in past. No deposit up front and you paid when you checked in ... guess if you can't get in due to weather, your reservation would just cancel out, but would check with them in any case.
We were in Cayman's on 12/11 and booked with Moby Dick's this time. I'm really impressed with them. No upfront deposit just bring copy of the e-mail reservation return. We met their representative on the pier as instructed. He advised us that it was VERY ROUGH out on the sandbar that day and he wasn't sure how much fun we would have. We told him we wanted to go anyway ... Rough is not the word for it. We were totally drenched before we were even within sight of the sandbar. The boat had maybe 50 passengers on board - mostly in the 20-40 age group. When we got to the sandbar, there were already about a 100 people in the water and most were having trouble standing up. When we finally anchored, the began to let our group off the boat. One woman stumbled getting off the ladder due to the waves and the captain immediately got all his passengers back in the boat. Too dangerous and no fun ... he suggested moving over to the snorkeling site as the deeper water might make it more fun ... after observing there, he took us all back to the pier and gave all of us a refund of our entire fare. We gave him a generous tip even though we had decided we would not get out of the boat due to the waves ... we were impressed that he was more concerned for the safety of his passengers than in making a profit for the trip!
Since this was well before 11 a.m., we have plenty of time for port shopping - not great as the $$$$$ are exretemly high.
When we go the next time, we will try Moby Dick's again due to this experience.