You don't say what sort of cat he is, or how old - some breeds (Bengals in particular) are well know for liking and playing with water - in which case you need to be vigilant about the bath!! And the loo.
If he's a kitten, he may well grow out of it, particularly if he has other things to play with.
Or just put down a plastic mat and let him carry on playing! My cat used to love lying in the basin and catching the water drips. Some cats just enjoy water!