Most of your questions can be answered on Splash Divers
website. [user]Ralph Capeling[/user] posts siting info here:
http://www.scubaboard.com/forums/belize/478615-whale-sharks-2014-a.html - he's one of the owners. All that activity takes place at Gladden Spit nearby. Some of the Hopkins/Dangriga area resorts will go there also - and some of the south Belize Atoll operators as well. None are particularly close but Placencia at least is a fairly straight ride out.
Here's the dates you have left:
May 14th to May 23rd
June 13th to June 22nd
Scuba with Belize Whale Sharks|Scuba Diving & Accommodation|Splash Dive Center|Placencia Belize Some friends dove with Splash - on two trips they dove with 8 sharks total. Should you want to dive the Blue Hole, Elbow and LighthouseReef, Splash does it as an overnight:
Belize Blue Hole and Whale Sharks | Scuba Diving & Accommodation | Splash Dive Center | Placencia Belize They also have an option to stay on Ambergris and dive from there. Boat ride is about an hour shorter. I don't know of any Ambergris operator that goes down to Gladden Spit though - I think it's too far for a day trip.
The problem with the Atoll based operators is they're typically 7 days on the atoll since they only shuttle guests back and forth once or twice per week. So for the Mayan ruins, pick something land-based. Splash also does inland tours - Mayan Ruins, Cockscomb Preserve, Monkey River etc.
Seahorse Dive is also a Placencia option if Splash is booked.
Belize Scuba Diving and Snorkeling In Placenica | Seahorse Dive Shop I saw somewhere Avadon Divers recently closed. They
were the 3rd Placencia option.
There's an Aggressor and a Dancer based out of Belize City. They dive Lighthouse, the Blue Hole etc. but do not
ever go south far enough to encounter whale sharks. I've even called to ask why but there was no real reason given. You'd think they'd be all over that.
Find more info in our Belize sub-forum also:
http://www.scubaboard.com/forums/belize/