Placencia has some really great beaches and some excellent dive centers, but I don't remember seeing any over the water cabanas. Lots of cabanas on the beach, though. Going "big" might mean staying at Francis Ford Coppola's Turtle Inn. You can even get Francis' private cabana, assuming he's not staying there when you go, complete with his butler.
Down in Punta Gorda, Dennis Garbutt built two cabanas and a tri-plex that jut out over the water at Garbutt's Marina. But these are pretty basic, nice but basic, and there's no beach at all there, just rocks. Great view of Honduras Bay and if you're lucky enough to be there during a thunderstorm you'll be treated to quite the light show. Dennis runs a small dive op out of the marina, but I haven't used it as he can't take out just one diver.
More "go big" choices: take a look at Turneffe Island Resort, Turneffe Flats and Blackbird Caye Resort, pretty much in that order. All are on the atoll. Private cabanas, right on the beach with in-house dive ops. Further south is Hatchet Caye with the same setup . Same is true for Hamanasi in Hopkins on the mainland but it's bigger with more of a hotel feel but does have some private cabanas.
Maybe Amergris Caye? I haven't been there but it's the most popular tourist destination so hopefully someone else will chime in.
