Horses are so different in how they like water!
My little arab sticks his nose underwater and blows bubbles. He paws, but reallly gently. More like he is stirring the water, and he sniffs it while he does it, as if he can't quite figure out what it is... It's so cute!
My friends mare used to attack the water. She would paw so hard you could see the bottom. She never rolled though.
I had a big stockhorse whow ouldn't even paw. He would sniff the water, and then whoof! He was laying in it and rolling. We used to let him loose with us when we went swimming and he would go lie down and watch us with his head sticking up our of the water :]
We always stand our horses in the water for ages with our feet trailing in the water. The only way to see what will happen is to let it happen :]