Saw them few times...
First time, diving, 21m down, 208, Tubbataha, liveabord in the Philippines.
Second, diving, 20 meters++, Hin Muang, 2009, liveaboard in Thailand
Third, saw tens of them snorkeling in Djibouti, January 2010.
Fourth, snorkeling with them, Tofo, Mozambique, November 2010.
Few more places were you are pretty sure to see them...
There are few places in the Philippines, but check the place and season...Liliram has writtne about it.
Ningalloo reef, way north of Perth, Australia around may.
Few places out of Los Cabos, Mexico...not sure fo the season, but think it's January.
South of Belize, but nothing to do with the Reef Barrier.
It's easy to see them while snorkeling as they are pretty predictable. Seeing them diving, you've got to be lucky.
I have a pic with a manta and a whaleshark on the same picture in Hin Muang, that was something!