Travelled to Belize a few years ago especially for the Whale Shark Aggregations in Gladden Spit.
The best place to stay as a base is Placencia a small town / village that is about a 60-90 min boat ride away (depending on sea conditions) from the protected marine park.
The trips a usually day trips with 1 dive in the morning and 1 early pm on the site. They limit the number of boats on site to 1 hour slots. In between dives the boat usually anchors on a reef nearby. Divers are free to snorkel during that time
Important to note that although comfortable the boats are usually small with no toilets.
The dives themselves are blue dives and basically can be hit or miss. On some dives we had up to 6 whale sharks, dolphins, Bull sharks, inquisitive raymoras and thousands of snappers and on a couple other dives, just deep blue dives....
It was however a fantastic experience.
I stayed in Placencia itself at The Turtle Inn (owned by Francis F Coppola) and used their dive shop which was great with few divers on each trip. Some other operators on larger boats might have a mix of divers / snorkelers...
Placencia itself is a fantastic village that feels relatively untouched by tourism (they have managed to keep the Cruise business at bay so far). Local people are very friendly, and there are some very nice local restaurants.
To me the town had a typical carribean vibe without the usual downside re safety etc... People can walk around the place at night without a worry (at least when I was there...)
Hope this helps and don't hesitate to let me know if you have any specific questions