I have not personally used them but there are many people on this board that have. Captin Marvin's name seems to come up alot and I haven't seen any bad things said about them. Same can be said about Moby Dick's tours but I don't see them discussed as often. You should be fine with either. If you want to read the posts about them, enter Captain Marvin in the "Find" tool and you will see the posts and can read them if you like.
Sonny is in the middle of a cruise so he may be delayed in answering your question. But don't be afraid to book now. The tour operators have dealt with the cruises for a long time. They will take a deposit now. They will refund it if for some reason you ship does not go to GC (say you run into a storm that forces a change in ports) So if you know the tour you want to take, go ahead and book it so you can be certain you can get onto it. If you wait it may fill up. Just be sure you book the correct date and get a confirmation email. When we booked a Belize tour they put us down for the wrong YEAR. I didn't notice because I only paid attention to the day and month. Luckily they were able to put together a crew and do our tour (while we stood on the pier)
We booked our cruise late Dec 06 for Apr 07. We booked the tour with Capt. Marvin end of Feb 07. It was definitely great and even our 7 y old DD enjoyed the sting rays. Capt. Marvin only wants your CC number to put a hold on your CC. For payment they prefer cash (they add 5% for CC payment). Their office is just a short walk from the pier. They donīt charge your CC in case the ship does not stop/tender in Grand Cayman.
Thanks so much for the quick writing back. I think I might book that time that why I am for sure going to make it there and still have enough time to make it back to the ship in time. Have you ever seen them leave anybody behind?