Hi. Sorry I can't answer your questions, I just wanted to suscribe to this thread and don't know how to do so without posting in it.
I'm leaving for Mexico and possibly Caye Caulker in a month, so would love to know more also.
FWIW, I dove in Mahahual in Jan.-Feb. 2004 and stayed three weeks there. That's actually where I got certified, so didn't have a way to compare the reef with any other. It did look pristine back then (huge gorgonias, nice corals), the vis was excellent, saw turtles, huge eels, lobsters, rays, barracudas and a wide variety of fish, but not in big schools.
I dove with Dreamtime and loved them, and stayed 4 miles south of "town" at the Travel Inn, a cheap, friendly place with a great little restaurant by the ocean. No local bus service, I just hitched rides to town or the dive op and it was very easy. Only had to catch a taxi a couple of times in three weeks (you can also rent a bicycle).
Great snorkeling from the shore too. Low key and laid back, except when the cruise ships docked (but they only stayed on land a few hours, and it wasn't a problem if you were staying out of the little town).
Please note that this was 5 years ago, before the town was hit by hurricane Dean. So it could all be different now.
Some dive ops in Mahahual and nearby Xcalak offer trips to Chinchorro Bank which is supposed to be awesome, but you need to have a minimum of divers signed up and decent weather to go, so it's hit and miss.
From my research, diving around Caye Caulker itself is not that great, but you can take trips to some of Belize's best sites (Turneffe, Aquarium…
. And the island itself sounds like a wonderful place to chill.
I'd love to do both too, but transportation between both places is either time consuming by bus and boat, or a bit costly for my budget by bus and plane.
I'll post a trip report when I come back in January.
Hope this thread gets more answers.
Anne-Laure