Last year I signed up for a "central Belize" cruise but because of high winds, Nekton, PH and Aggressor played musical moorings around Ambergris and Lighthouse reefs. Most reefs were okay but not great. Didn’t see any "barren of sea life" but that is what I had heard and why I wanted to go further south but we didn’t. Maybe our DM’s being on a liveaboard just had a wider selection of locations than yours did.
Sorry your trip was such a disapointment, that always sucks!
What bothered me on that trip was the trash on the beach and in the water around Belize City, the locals don’t seem to care. Yes I know Belize City is a slum.