The gentle giant Whale Shark is the biggest fish species around. I still have not had the honour of snorkelling with one, but I still hope!!
On rare occasions you might see schools of big fish, like here, grey reef sharks, passing by, ususally while channel diving. Fushivaru Kandu in Lhaviyani atoll is an example where this might happen.
Graceful Sting rays can be seen all over. baby ones live close to the beach, in the lagoon, together with very young sharks. No, they are not dangerous as long as you do not step on one, being stepped on could upset anyone, Sting ray or not. ;.)
...and the turtles!!! They are absolutely everywhere! Very sweet and cute, but do not touch! A turtle might be spooked and dive and drown if you touch them. Come think of it; Don't touch anything. Just enjoy the show!