That's why you normally have to wear a helmet with a face guard for canoe polo...
The other time people get hit with a paddle is when they are doing rescues. You get the capsized person to hold onto the front of your kayak, and if the have kept hold of their paddle you get them to pass it to you. And it normally arrives like a javelin! Eventually I learned to ask for the paddle and then tilt my head foward so the paddle would glance off my crash hat, and then I could take control of it and complete the rescue.