Right when you get off the tenders, there are a good number of operators. The issue is when you can get off the ship. If you book the ship's excursion, then you will be one of the first off. They use high speed tenders, as the water is so shallow so far out and the ships anchor up miles from Belize City.
Unless you have plenty of time in Belize I think I would opt to book a shore excursion on the ship. I you do have a full day in port, then maybe wait until you get on shore.
Just my opinion.