I have seen a Cooper hawk sitting on a low branch above my small Koi pond 3 times in 4 months. The books and net say they eat birds and some mammals but I wonder if he is looking at my fish ? Or snakes or frogs ? As I write this I had a thought, song birds come down to a shallow part of the pond, that may be what the hawk is after. But I still wonder about the original question.
Birds learn new tricks very often, using roads to crack nuts, opening foil milk bottles (tits), and Hawks are known to catch Koi.
These pages say that Cooper Hawks specifically do catch fish: http://forums.gardenweb.com/discussions/1997093/hawks-taking-koi http://www.outdooralabama.com/coopers-hawk
Here is a cooper hawk catching a duck and swimming around with it: https://www.youtube.com/watch?v=A08jxYinAG8
if you put cooper hawk pond into youtube, you will find 10+ videos of them bathing in ponds, and 2 videos of them scouting at a pond.
My aunt saw a buzzard, Buteo Buteo 50cm tall, catching a yellow Koi right out of the water while I was out for the day. The Koi was 30cm long, 12", it dived on the water and flew away with a fish on the wing. The Koi had indeed vanished as my aunt had said.