Lucky you Bobby, especially if you are a diver. I have not been to Belize (yet), but I have been to Roatan twice. There is not much more than craft market stalls at the ship's dock. I'm not sure about car rentals being available at the dock, but I don't think so. Avis does rent cars at the airport. I think a taxi would be a better bet.
The first time in Roatan, we walked off the gangway, and hooked with a boat trip going around West End. Here's the link to the excursion we took: <http://roatan-island.awwm.com/saltpepper.htm> The snorkeling from the small cove where the restaurant was was so-so. But the weather had been stormy and had clouded up the water. The best part of that trip was the boat ride and listening to Paul tell us all about Roatan, and the view of the island from the boat.
My second trip to Roatan, I took the ship's excursion to Tabayana Beach, along with just about everyone on the ship. Despite the high number of cruisers flocking to this beach, I have to say I enjoyed it very much. The beach is very long, facilities were very nice, and the snorkeling from the beach was quite good. The bus traverses the island and you have SPECTACULAR views of the ocean from the hill/mountain tops. I would NOT try to drive on these cross country dirt roads myself. A taxi will take you to Tabayana beach, but supposedly you can only get in there with a ship's excursion. That is probably just cruiseline hype.
I know there are other interesting places to go also...Fantasy Island for one.