Boat fuel is expensive, so whatever the trip there will be a quorum below which the trip is uneconomic. For example, no BH dive trip will leave with less than 10 divers on board. Some trips are less in demand than others, such as a dive trip to the Elbow on Turneffe, and it could be that at that time there wouldn't be the minimum number of potential customers.
PM me if you'd like me to help you with contacting dive centers or arranging diving. Make sure to give an email address. I run a dive center in San Pedro, though south rather than north. Where will you be staying?