Bathtime craziness

Ok mama's, needing some suggestions.

My son always pees and poops in the tub. It is getting to be very difficult as we have an old fashioned tub and the hot and cold are separate. So I have to remove him from the cast iron tub clean it and fill it back up again. I am also pregnant and having a bad aversion to water (specifically our hard water) so the whole time I am barfing. This evening I had to fill it three times. He was only in the tub for 2 minutes total. What do I do? He gets changed very frequently as he prefers a dry diaper and loves bathtime. I don't want him to hate it but it is so miserable for me.

Thanks for the help!