Let me explain my "free choice" comment. It isn't a dig at different operators in Belize City - far from it - but results from the cruise ship operators' practice of making it difficult for "cruisers" (if I may call them that!) to join tours not purchased from the cruise ship. They do this in several ways. They make it more difficult (and slower) for people doing their own thing to leave and later rejoin the ship. On your return you typically have to make an earlier transfer boat than is available to people on tours they've sold. And once the posted departure time arrives, they will wait for passengers they've booked on tours but NOT for people who booked independently.

I also had in mind that I have been told by people who tried to book independently with one of the two main operators (both of whom do an excellent job) that even though there were trips going (for the customers booked by the cruise ship) they were unable to join them. Whether the boats were full or the cruise ship had given instructions to operate restrictive practices I don't know (I really don't) but the bottom line is that they found themselves unable to get on a trip run from Belize City.

I've never been on a cruise and what I hear from passengers doesn't make me want to try it.

FWIW I would happily go out diving with Sea Sports Belize, a most professional outfit. But there's nothing wrong with "Belize Dive Connection" either.