If you are on the pill and take it regularly and correctly you would be as protected against pregnancy while having sex in a bathtub as you would having sex in bed.
As for the man wearing a condom the chances of it slipping are probably greater in a tub. Make sure he puts it on while erect and still dry and that he holds the rim while he withdraws after he ejaculates. He should also withdraw as soon as he ejaculates and before he loses his erection.
PS- Don't slip in the tub...that would make sex rather unsafe.