Thanks Ron and Dane.
I've got a Rough Guide that says that Protech Belize runs 2-day diving/snorkeling trips to the "outer atolls" for $275 per/person. There may not be an equivalent on CC, though.
It also mentions a chance to see "a number of previously inaccessible Mayan sites on the north coast of Ambergris Caye", and it goes on at length about the "thick jungle" and "sense of adventure". Can anyone confirm or deny this?
I fee like the Rough Guide may be a bit outdated - it was published 3 years ago, so the info may be up to 5 years old. Who knows what has happened since.