I have been to Roatan, Bonaire, and Cozumel. (Grand Cayman, too, but on a cruise, and I did not dive.) I say you cannot go wrong with ANY two of them! There is no wrong choice, even for a new diver. Looking at the decision from a different perspective, which of these ports of call offer activities other than diving that interest you? In other words, save the diving for the ports of call that do not offer much else that interests you. I believe Roatan has stuff for cruise shippers to see and do other than diving (though I've never done any!). Bonaire has, I dunno, the Donkey Sanctuary and maybe mangrove kayaking and windsailing. Cozumel, well, you could drink margaritas, listen to mariachis, and get hustled by touts selling stuff, but if I were you I'd go diving instead. The dive ops are all very close to where the cruise ships dock. The Cayman cruise hordes enjoy the snorkel with stingrays thing--I kinda enjoyed it. I can't speak to the diving, though.