Yes you can just walk to the beach in Roatan. Assuming you dock in Mahogany Bay (sometimes its too windy to dock so they use the RCL dock or skip Roatan altogether)
In Belize... there is no beach. You either need to book a ships excursion where they take you to one of the barrier islands they have turned into a beach or tender into the city where you can book a beach day thing. So a "few hours" at a beach is not practical at Belize.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007