I spent two weeks on Long Caye, Glover's Reef, Belize. There are two outfits on the island:
Off The Wall dive resort, and
Slickrock. The latter is an all-inclusive multi-activity program, principally sea kayaking, snorkeling, windsurfing, kayak surfing, etc. The former is just a dive resort. If you stay at Slickrock, you can dive, at additional cost. If you stay at Off The Wall you cannot join any Slickrock activities or use their equipment.
Both are great places run by wonderful people.
The diving is Long Caye Wall, a 5-minute boat ride away and all diving is from the boat. I don't believe there's any shore diving, though if you get certified there your "confined water" sessions are in the shallow water right off shore by the pier. Long Caye Wall is spectacular diving. My experience is limited, though, so I cannot compare it to much. Cozumel was wonderful, but did not come close. Because the dive sites are just minutes away, your surface intervals are spent on the island, unless you go on the extended whale shark dive trip, which I believe is seasonal.
There's nothing else on the island but coconuts. Both resorts provide all meals. Accommodations are very rustic. When I was there (February, 2008) insects were not a problem, though you should have bug repellent just in case. Sunburn is definitely a hazard.
I'm going back this December. I'll be staying at Slickrock because I want a variety of water activities, and one dive a day works well for me. But if you want to just dive, it will be cheaper to stay at the dive resort. They have several different packages. Jim and Kendra are wonderful people, and my instructor (Lucky) was infinitely patient with me. (I got OW certified there.)
If you stay with Slickrock, they'll transport you by boat from Belize City, 2 1/2 hours. I did not enjoy the boat ride out and back, but that's the price you pay. Off The Wall uses a shorter and more direct boat ride, but you have to get yourself to the little town they depart from.
If you can handle the rustic accommodations, the island is paradise. (I liked it enough to be going back.)
Daniel