Tabyana Beach is in Roatan...I rode by there but did not stop when I was at Roatan last year. I understand they have a very nice snorkling area, but also a nice beach.
In the past, there was a place called Bannister Island in Belize, but after the issues Carnival had with the Belize Tender operators, not sure if people from Carnival ships are going there now.
But, the good news is Carnival is going to Belize again and seem to have settled with the operators. If you see that available, I know some who went on that trip last year and they said Bannister was wonderful...nice beach and also fantastic snorkeling.
The great thing about both locations is that they are on the second longest reef in the world..and both have some very good locations for diving or snorkeling.