Hey there !
I worked in the Seychelles for GVI, a marine conservation organization and we were based on Mahe, the main island, and with no hesitation I'd recommend you to dive with 'the underwater center' http://www.diveseychelles.com.sc/
. Not only Glynnis & David, the owners, are professional & friendly, but David is also the founder and director of MCSS, the marine conservation society of the Seychelles, which amongst other activities, monitors and tags whale sharks. Yes, the Seychelles are one of the hot spots for whale sharks, and with the right timing, you might be lucky enough to see many. Make sure to get in touch with them before hands to get further details.
During the peak of whale shark season (generally from september to december, but there's a short period around june/july when they suddenly appear and gather in huge numbers), MCSS / the underwater center uses an ultra light spotter plane to search for them and you can join the monitoring team on the boat and jump in with the animals, and you can even arrange an unforgettable flight with their spotter plane.
The Underwater Center is based in Beau Vallon, on Mahe's west coast, which offers the best diving in the area.
The Seychelles are granitic islands, and dives close to the shore offer very unique reefscapes, with massive granitic boulders on which generally coral growth is limited to relatively small colonies, some of the sites have better coral growth though, such as the Baie Ternay marine park.
Make sure you visit some of the offshore sites which can be taming with life. Out of memory, the one site that comes to my mind is Shark Bank, a 20mn ride or so from the dive center, heading straight out from the shore, which use to have a big school, of chevron barracudas, snappers, eagle rays, big stingrays, white tips, nurse sharks, amongst many other species.
The average visibility over the 3 years i spent diving in the Seychelles is commonly 30 meters +, absolutely fantastic conditions for photography.
One last thing, i would recommend you to try to stay at a small family owned hotel / B&B, which offer much more affordable than the big hotels or resorts and a Seychellois cultural experience as well. Also, make sure to visit other islands as well, Praslin, the second largest island has a total different spirit to it and offers some good diving too, and La Digue is a must for a chilled few days, with only a couple of cars and everyone riding bicycles.
I'm gonna stop now as i could go on and on about the Seychelles, which also offer exceptional topside scenic views. Feel free to contact me for further details, and yuo can check my flickr page for some photos of the Seychelles UW & topside: http://www.flickr.com/photos/hamidrad