I don't know much about cruises, but I can say that the barrier reef in Belize is spectacular. It's a very beautiful area. They've had some politicial problems this year, so you may want to check into booking there.
One thing I can tell you about fishing in Cozumel is that you may want to consider fishing for bonefish. There are guides for trips like that and they aren't terribly expensive, compared to other guided trips. The fish run relatively small, but they are abundant and are great fighters. There are other reef fish. In June, I've caught bonefish, barracuda, and baby tarpon in abundance. It's a lot of fun. I prefer fly fishing for the bonefish, but spinning works too. Try very light jigs tipped with squid or similar cut bait. Plugs on a baitcaster are the most fun for the barracudas and tarpon.
Most people who fish Cozumel are after the offshore fishing, but the inshore fishing is some of the best I've ever experienced anywhere.