We just did the Sun Dancer II last month. I can't say enough good things about the vessel, the crew, the rooms, etc. Although, the food isn't as fancy as they advertise it to be. This was not my first liveaboard, but it was the nicest vessel I've been on by far. We took a tour of the Belize Aggressor III when we returned to port. It looked like the Sun Dancer's crackhead cousin in comparison....and it was much more expensive than the Sun Dancer II, especially since we went on a 25% off Buddy Week.
The diving in Belize....wasn't anything amazing IMO. Everyday was pretty much the same without much variety. There were usually two wall dives and two shallower above-reef dives + a night dive. The first 2 dives of the day were the same dive, and the 3-5 dives were the same dive. The Aggressor was almost always near by at the same location, with the exception of the day we went to the Blue Hole.
The viz was not the best. At some times, we were only at 20-25 feet and couldn't see the boat above us. But for the most part, it was around 50, I'd say, for most of the trip. There were a ton of macro critters, which I love to see because I like to practice with my camera. As far as big guys go...we did see a few Eagle Rays, quite a few Stingrays, a few people saw sharks (but we missed 'em), lots of Moray Eels, etc. The best part of the dive is taking a ride on the hang bar at 15 feet!! I didn't touch it until we were about 4 days into the trip because I'm a fairly independent safety stopper
~ but it's fun to hang on to when the boat is really thrashing from side to side.
Overall....I love being on a liveaboard more than anything in the world, loved the crew, loved the vessel, and we had a great crew of divers on board...so I would definitely go again. But I'll never brag about the diving on that trip. Don't get me wrong. It wasn't bad diving. It's just not what it's all hyped up to be.
Hope that helps a little.
Just my $.02