I was hoping to see a reef shark or a turtle but unfortunately it wasn't to be. I did get to see turtles in a rehabilitation faciity and at Fitzroy Island though. On the reef itself I saw many varieties of smaller fish and a many of Nemo and Dory from Finding Nemo
They'd swim right up amongst you and infront of your goggles which was super cool. The biggest fish I saw was a Maori Wrasse named Wally who apparently is always hanging around the pontoon I visited.
Did you manage to get out of the airport there? The esplanade is interesting because half of the day it is water up to the boardwalk and the other half of the day it is just mud flats for a long way out. If you were only there on a layover I imagine you probably only saw one or the other?
Something else I really liked about Cairns was that there are so many Japanese people there so the Japanese food felt really authentic. It seemed like nearly all shop staff there spoke Japanese and shops were advertising that you can pay in Japanese yen in their windows which I've never seen down here in Sydney before.