D.P.
my 20 month old has been doing the same thing. I ask her to sit several times and then tell her we are all done with the bath. she sits. but, if he wont sit after a few threats, wash him down quick and take him out. he will probably fight it, but will learn he needs to sit in the tub or he doesnt get to be in there. i assume he likes the bath and will learn what behavior allows him to stay in and play.