Skillfully and effortlessly, as if my sister were a feather, he grabbed her, along with her paddle, with one arm and threw her overboard.

That was unprofessional and dangerous - a very good reason to call him an asshole for that move.

However, your later action and the result,

We grabbed both of his arms and pulled them down toward the water, and he almost fell. But he was far too strong to be tipped over that easily, so, once he regained his balance, he grabbed both of us by our life jackets and dunked us into the water.

I can only comment with
:o)