My friends and I got a x3 session deal on a new flotation tank place back in the early 00s.
It's basically exceptionally concentrated magnesium (Epsom salts), far more than you'd manage with a bag in a bath.
It was so concentrated You'd float for an hour in the tank.
We'd have a meal together afterwards at each other's houses and I do remember floating around Sainburies in a daze. It used to make us dopey and we'd sleep really well. Didn't bloat back then so can't comment! Don't remember feeling dehydrated.
So I think a bag in the bath is fine. I e often felt a cup is too little. I now use home made spray (different type of magnesium) and spray all over a couple of times a day.
If anything a hot bath for too long can be bad for blood pressure. The tanks were body temp.