Based on our last visits to Roatan, Cozumel, and Costa Maya (sorry, we have not been to Belize); I would recommend:
Roatan: Roatan Cruise and Caribbean Cruise Shore Excursions | Maya Key
Cozumel: Nachi Cocom Beach Club and Water Sports Center | Cozumel, Quintana Roo | México
Costa Maya: We have always just hanged-out in the port area. Very nice pool, food, and stores.
FYI: Cozumel Beaches; an interactive beach map.
05/89 NCL, Seaward, 1990 Carnival, Topicale, 1991 Carnival, Festivale, 11/93 Wind Star, Wind Song, 01/95 Celebrity, Horizon, 01/96 Celebrity, Century, 01/00 Celebrity, Zenith, 01/01 Princess, Ocean Princess, 01/02 Princess, Ocean Princess, 01/03 Celebrity, Summit, 05/04 Princess, Diamond, 01/05 Celebrity, Galaxy, 01/07 Oceania, Regatta, 01/08 RCCL, Voyager of the Seas, 02/09 RCCL, Voyager of the Seas, 01/10 Celebrity, Solstice