I guess this is all in perspective though, I still think they turn pretty realisictly but given the quality of the other swimming movements and the quality of the models, it stands out.
Of course, I seem to be the only one seeing this
No, you're not the only one seeing this, I've noticed it for some time as well. I neglected to bring it to Jim's attention because I thought it would be cleared up when he began making final AI and animation changes to each of the different species. Basically, I just chalked it up to unfinished work pending completion, but it is noticable and obvious for anyone that understands how fish swim, turn, and move through the water.
It is very difficult to describe and it is random, but what I'm seeing is a fish will turn part of the way in a realistic manner (head moves through the path with the body and tail following), then the last part of the turn the fish looks more like it's sliding on a moving pivot (instead of the head leading the way, the middle of the body follows the path). In any case, you just have to watch the fish for a while, watch as they turn near the sides of the aquarium, eventually you will see it.