I've been to Belize twice, Bonaire twice and Dominica once. Each are different and enjoyable. Bonaire is probably the most "boring" of the three in that the topside landscape is not that interesting and water activities are mostly the only thing to do (diving, kiteboarding, windsurfing, kayaking the magroves). It's easy to get to for a single week trip. The diving is very easy with complete time freedom with shore dives. Lots of little fish, not much in the way of larger fish. All dives are generally good, none are usually great. The UW terrain is mostly similar site to site, a 45 degree sloping reef. Bonaire does have the Hilma Hooker wreck.The easiest and the cheapest of the three to get 20+ dives in a week, but with the dive sites so close in both Belize and Dominica 3 dives a day at those two is also easy if not more expensive being boat dives. Bonaire has lots of good restaurants. Wide variety and lots of good places to stay.
Ambergris Caye is all boat diving but the reefs are very close so you can do two dives and be done around noon, plus most ops come back to shore between dives. Diving is good but probably not great, but you will see bigger fish - eagle rays, stingrays, turtles, grouper and nurse sharks, compared to Bonaire and Dominica. And on a dive that you see a bunch of big stuff I find that more interesting than the little stuff on the other two. The UW terrain can be interesting with spur and groove reefs and some smaller swim throughs. Lots of good restaurants and I find the funky San Pedro town more interesting than Bonaire. It also has a wide variety and lots of good places to stay. It's also easy to get to for a week. Interesting mainland activities that can be done as a single day trip if you give up that day's diving. And the interior jungle is really great. We did the ATM tour near San Ignacio and it's one of the best things we've ever done. If you have some extra days to stay inland then Tikal across the border in Guatemala is fantastic, also an all time trip highlight for us . Ten days in Belize spending some time both inland and diving on the Cayes would be a nice trip.
Dominica is the hardest of the three to get to. At least for me coming from the west coast I wouldn't put up with the travel hassle for a only single week. But if I had two weeks, maybe even just 10 days I would go to Dominca over the other two without question. The diving is just as good, similar to Bonaire in that it's mostly small fish. The UW terrain is much more interesting, shallow reefs, shallow walls down to 100 feet, deep walls down to 400 feet and pinnacle dives. But the interior jungle is where Domina really shines, it's absolutely fantastic. Dominica gets 300 inches of rain a year, basically it rains ups in the jungle almost every day somewhere. Lots of rivers and waterfalls all over. The jungle hikes and waterfalls are great. We did canyoning with Extreme Dominica and it was one of the most fun things we ever did on a trip. We also did the boiling lake hike, also really cool. It really does boil! The food, island wide, is mediocre at best, do not go expecting great meals. Only a small variety of places to stay with Ft. Young maybe being the only really nice place to stay while diving. Lots of nice jungle lodges to stay at.
We plan to go back to all three, depending on how long we have and what we are looking for that trip. Only a week and wanting to do lots of diving and lots of relaxing then Bonaire may win out. A little more than a week and wanting a varierty of jungle and diving, and/or hoping to see bigger fish then Belize may win. If we had two weeks then Dominica wins with 4-6 days in the jungle for some serious jungle hiking and the balance at the water diving.
Topside and diving photos, and other info of our Dominca and Bonaire trips can be found on my website
www.diverrex.com. Sorry but I didn't have an underwater camera during my Belize triips so it's not on the site.