We just got back from Ambergris Caye and I couldn't recommend it more. Yes the beaches are public, but isn't part of the point of visiting another place to interact with the people there? If I need to be insulated, I'd sit home on my couch in my own private world.
Ambergris was different then anyplace else I've been in the Caribbean, but in a really nice way. I loved the laid back natural non commercial feel of it, the 'no worries' attitude there, and the diving was heavenly. As two females traveling alone we felt perfectly safe at all times, and loved that we could actually go downtown and enjoy some of the local places without being told we had to be back at our hotel before nightfall or it wouldn't be safe (which we have been told at other places we've been). We stayed at the Banana Beach Resort which was a little far from town, but no problem to get picked up there for diving or any other activities. If I go again I'd probably stay closer to town just so we wouldn't have to do the long walk back at night in heels
We dove and snorkeled with Ecologic Divers, and they were excellent. We did day and night time dives with them, and loved both. I won't bore you with miles of details, but my friend who wasn't even comfortable in the water before going out with them is now taking swimming lessons, and is determined to be a certified diver one day. In 31 years, no one has ever made her feel that comfortable or that excited about being in the water, so to me that says a lot about how good these guys are.
No matter where you go, I really don't think you'll regret choosing Belize. I've been a lot of places, lived in other countries etc, and there's never been somewhere I've wanted to go back to more then once. I am booking a trip back to Ambergris Caye this week.
Good luck with your choice, and no matter what you decide, make the most of every second because it will be over way too fast.
Carol