DS went through this about 2 months ago. He is now 18mo LOVES his bath but went through a stage of spending less and less time in the bath before refusing to get in at all, crying if he even touched the water. We don't know what set him off, could have been getting his hair washed or slipping on the surface of the bath.
We ended up filling a little washing up bowl with warm water and bubble bath and putting some of his toys in. We put towels on the floor of the bathroom so he could splash if he wanted. At the start he wouldn't put his hands in at all but played with the toys and I managed to just top and tail him. Finally he started putting his hands into the soapy suds and once he was OK with that we began putting the washing up bowl into the empty bath so he would do the same thing in the empty bath.
He started showing curiosity about the taps so as each evening progressed we added a little warm water to the bath as he sat in it, always starting when he was sitting in it empty. Little by little we added more water each evening and eventually he went back to the usual routine and woul get into the bath when the water was in it.
For us the key was trying things really slowly and gently - it took us about 6 weeks to get back on track but I honestly don't believe we could have done anything different.