If you want to do a Belize liveaboard, consider
Sun Dancer instead. They and Aggressor are both owned by the Aggressor people (Hughes sold out a couple years ago) and you'll save quite a lot of money on the Dancer - as much as $600. I think it's also a bigger boat. They run the same itinerary basically - compare the maps/descriptions on their websites.
Don't expect to see any whale sharks on their trips as they go nowhere near where the whale sharks are - off Gladden Spit in south Belize.
They do run to some nice areas, the Blue Hole, Lighthouse etc. so it's still a good trip. Also it's convenient as they'll pick you up at the airport and take you directly to the boat so no logistics problems or overnight stays - assuming you can get there mid-afternoon.
If you want to dive with Whale sharks you'll either have to shore base in the Placencia Area (Splash Divers, Avadon or Seahorse) or stay at one of the resorts in the Hopkins area or dive resorts on the atolls - IIRC Isla Marisol or Thatch Caye - among others - who do trips down there during the week. Land based on Belize isn't the cheapest though once you add in the flights from BZE to those areas. Most of them also charge in the $150+ range for the trip to Gladden Spit also. We were going to do it last year but couldn't make the numbers work.
I was going to suggest Buddy Dive/Bonaire as an alternative. Bonaire is one place that I've been asked multiple times if I needed a buddy while standing near the dock/rinse tank waiting for mine to arrive. The de-facto dive package vehicle is a 4dr. pickup so people usually don't mind if you join them for dives when asked. Just about everybody we met there, on the plane going there, at many of the bars - was a diver.
The downside of it will be if you don't find someone you'd have to boat dive with Buddy's - they'll put a DM in the water and try to pair you up also - since you don't want to solo. It's probably not the best idea to solo on Bonaire anyway, there's a lot of ironshore/coral rubble at most entry points so it's fairly easy to go down and hurt yourself. Esp. in the afternoons, at many sites we were the only ones in the area.
You could also "dive the resorts" - many of them are on named dive sites. Check in with the dive operator so someone knows you're out there. Bonaire diving couldn't be simpler, the vis is generally excellent, the reef parallels the shore in most areas and it's usually very low current diving - except at sites in Slagbaai Park or south around the Lighthouse - those dives are marked advanced on any dive site list.
Another option might be diving with the
Dive Bus on Curacao. Same diving as Bonaire except the shore dives are more spread out. Many of the dive resorts/hotels have affiliated dive operators on site for boat dives. Several have shore dives off their property also. It's probably easier to get there than Bonaire also from CO - it is for us.