We went through two bath-aversion phases with our daughter (now nearly 2 1/2). The first time was at about 18 months, and I read somewhere that at this age children are noticing how the water goes down the drain and they may become afraid of the drain. We combated this phase by taking baths with her, and though she usually sat on our laps during this time, at least she got a bath! And she got over it after a while.
The second phase was several months later, and I still don't know what caused that. It even transferred to swimming this past summer, where she was totally afraid to get into the pool! However, my husband bought her some bath markers and that _totally_ did away with any fears she had. Immediately she happily got into the tub and had a great time. So I think, in our cases, it was a developmental issue where she was suddenly becoming more aware of dangers and weird things she hadn't noticed before, and it was a little overwhelming. Now she loves baths, and we can't even say the word "bath" around her or she gets way too excited.
Hang in there, she'll get over it, just be patient and work through it!