Hey A.,
I have a feeling it's a fairly normal thing -- he's learned to roll over, and it's fun, and he's also learning how you respond when you get frustrated with him, and it's great experimentation. He doesn't see it at frustration, however, he simply sees that his behavior brings a reaction. and that's very interesting.
What I would do ? Hold his legs by the ankles, and up in the air with one hand, and clean him with the other. Be prepared with everything you need in advance so you don't have to reach too far to get anything. I think once he doesn't win, he'll discover that it is more fun to get the job over with. It may still take muscle, but you can consider it part of your daily work-out. (yeah, right :-)
In the tub ? Well, keep the water level low enough that he can enjoy his new found ability and be sure to stay close by. I thought staying right there was "normal" but today on the news there's a story about a 7 month old who almost drowned in the tub, so I wonder who thought a child that young was safe without an adult right there all the time?
Good luck. I think you'll find that this is temporary, at least to some extent. When he becomes more mobile in other eways, he'll do different things. But right now, he has this new skill and he wants to do it -- EVERYWHERE !! As well as give Mom a little challenge to her life.