My cousin can't swim so I threw him in the pool.
You're kidding me, right? Why did you throw your cousin in the pool?
Because I do enjoy their expressions of hopelessness and fear.
It's not ok to throw people in the pool. They could drown or at least be seriously hurt. Better to just let them get into the pool themselves.
I'm sure he will either stand up or hang on to the side.
You aren't sure what he would do in the pool so it is best not to assume. It is reasonable to be angry when your family throws you in the water. You should expect him to be upset with you afterwards.