It's unfortunately, now or never for you my friend.
You must take a stand now, or weaseling your way out from under her clutches may never happen.
Tell her you two can do something else together, what that is idk, but make it clear that the pool is your sanctuary, and that of course she may go to the hot tub, you're not forbidding her, its just that she can go on any day except Fridays...
Your mom is unbelievable