Some children are content to sit in a tub while you wash them quickly and get them out. Others want to be in the tub as long as you will let them (or until the water runs cold). Many children, however, want nothing more than to throw a fit if you try to bathe without them. Sometimes that's because they want to bathe too, but I think, more often that not, it's because you've shut them out of the room you are in.
No matter what scenario sounds the most like your child, there is a possible solution. I understand this isn't a solution for literally everyone, but this might help for anyone who has a child that is a skilled sitter (up until you aren't comfortable with it anymore).
Both of my children fall into the "I want to be in the tub every possible waking moment" category, though Squiggle was like that and she freaked out if I left the room for a millisecond. The best way for me to successfully shower and the children to be happy was to shower with them on the other end of the tub. Bobble, being the crazy kid he is, loved being IN the falling water if I let him.
I would grab a few bath toys, start up the shower so it was nice and hot (the water cools as it falls, so hot to me is luke warm by the time it reaches them), and climb in with baby in tow. I would plug the tub and allow a bit of water to collect, then drain it when it got a bit high. I'd repeat until I was done showering, then I'd bathe the munchkin (after all, they just sat in water that was rinsing dirt off of mom!), and we'd get out at the same time. That usually resulted in a happy baby.
Also, we have one of those sliding track doors on our tub, so there is no place to sit to bathe baby from the outside, anyway.
Disclaimer: Thoughts of Fluff was not compensated for this post. All opinions are my own and may differ from those of your own.