Do Cooper Hawks eat fish?
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.