They can do polo on land. They can throw things.. believe me. My pony picked up my grooming box and threw it on my toe. So they could probably, if the water was not deep, swim and throw things. They could also throw humans off of them in the water. I have seen it happen.
Do you not know what water polo is? Plus its kinda the humans in this case that are doing a lot of the "polo"ing. Its them maintaining control of their horses to hit stuff, not the horse doing anything besides moving.
