Could be a muscle thing. If your horse has stiff, or tight muscles and when he tenses his body reacting to cold water, it might make him cramp. What I would try doing, is sending your horse into the water without you (if possible), get a lounge line or a longer lead rope, and send him in. If he has a spell, allow him to do so. But try and not to let him out of the water. If is is a muscle cramp type problem, he needs to realize on his own it's not the water's fault. That way he doesnt associate water with pain. Allow him to buck and get tight and so whatever his body needs to, and see if on his own he can come out of it and relax himself.
Do you notice any other muscle related problems with this horse?
It sounds like the water makes him tense up, and then bucking happens because he hasnt figured out to come out of it. If you keep him in the water even though he might be having a spell he'll relax in the water, and not get in the habit of running away from the water to relieve the pain.
By turning him in a circle and kicking him, it makes his muscles constrict even more so. Right idea, though doesnt sound appropriate at the given situation.
It could be something entirely different, but try it out and see what happens. :) Hope that helped!