Yes, there are outfitters who make the run to Lighthouse reef & Blue hole from AC. Do a search and you will find many names.
The best way to get to Placencia is flying from BZ City. There is also daily bus service but it is a fairly long ride. Of course you could rent a vehicle in Bz City and drive. No direct way from AC.
For the whale shark dives, take a look at Seahorse Divers in placencia. www.belizescuba.com.
They book up quite far in advance so make your plans early!