Caye Caulker diving - definitely Belize Diving Services, they don't just stick to local diving (though sites around sp and cc locally are awesome) - they have several boats and do run out to both blue hole, turneffe north and south and also out to Lighthouse reef without the blue hole. I have worked on the Sundancer and yes the sites they go out to are fabulous and you are pretty much without fail going to be the first dive boat out there, the boat is fabulous and service second to none. I worked there with Anne Marie McNeill who now owns Avadon Divers in Placencia (won the 2011 Belize Tourism Board's award for "Best Tour Operator of the Year,") - your best bet for going to Glover's to see whalesharks.
here are some contacts:
Chip or Dani Petersen at Belize Diving Services, Caye Caulker
Anne Marie & Phillip McNeill at Avadon Divers Placencia
and I agree with the other posters regarding "cheap" - it's a competitive business here and if one shop is cheaper than the others then ask yourself "what has been compromised?"..... would you go for the "cheapest" skydiving outfit in a competitive market???