We have been offering ship excursions for several years now and service all major lines coming to port. Your experience will depend a lot on the weather. The time slot is around six hours giving us plenty of time to pick you up, gear you up, go diving, and get you back to the ship. We offer Turneffe Atoll, which is within half hour from our shop. We can always find a leeward side over there as well as some of the best diving around here. The Barrier Reef is only 10 minutes away from our shop and is in better shape than up North. However, due to prevailing southeasterly winds (and strong northerlies during cold fronts) we cannot guarantee good diving on any specific day in advanced at the Barrier Reef. Our service is personalized and professional with top-of-the-line custom dive boats without the "cattle boat" feel as others would perceive. You can Google "cruise ship diving Belize City" or anything in that vein and get several results with links to operators offering you Barrier reef dives for less. The Barrier Reef is a hit or miss dive. When it's good it's excellent but when it's bad it's horrible.
Take all the time you need, do your research and enjoy your Belize diving.