Best island/caye for swimming and snorkeling

We are doing the Western Caribbean cruise with a stop in Belize. We want to just go to a beach for the day to swim and snorkel. We were considering Goff's Caye but in reading about different islands, we see some cruiselines have direct access to private islands. Such as, Carnival seems to have their own tour to Bannister Island. Does anyone know if Celebrity has a tour to an island?? We have looked at the tours offered on our cruise and nothing is offered to an island. But we are still 11 months away. Will something be added to the tours list? We like the idea of being picked up right at the ship to save the extra 30 minute tender to the port and back. Thanks for any info you can give us.