Belize District is Belize City, Ambergris Caye and Caye Caulker: airport, water taxis and the northern Barrier Reef. If you're diving in the north, you're here. The city is a transit hinge, not a beach holiday. Get onto a caye the same day if you can.
Island diving and the mainland hinge
Fly or taxi to the cayes, then let wind run the diary. Hol Chan, Mexico Rocks and atoll days when the east wind lies down. Scuba courses and fun dives are the volume. Snorkel the same sites if you aren't certified. Blue Hole remains a maybe.
Getting inland
Buses and shuttles to Cayo leave from the mainland, not from your dock. Plan a night that isn't a 4am water taxi plus a cave swim. Open the Book here tab for reef days in Northern Cayes and Belize District.