She may have had a bad experience with water or a bathtub -- one of my kids became terribly scared of the water after he slipped in the tub and went underwater for a few seconds. He grew out of it, but for a while it was difficult to give him a bath. She also may have watched a movie or TV show that scared her, in which somebody went down the drain, or a scary monster came up out of the drain (even a funny TV commercial about germs in the bathtub or drain could be scary to such a small child).
I would suggest minimizing baths -- there is no need for her to take a daily bath unless she gets dirty. Wash her hands and feet as needed; use wet wipes on her diaper area, and she'll probably be fine with just a weekly or twice-weekly bath. (In fact, daily bathing can irritate skin conditions like eczema; and there are a lot of doctors who think that we are now "too clean" and that our lack of exposure to normal dirt is causing health problems like allergies and stuff.)
She may be terrified of hearing the water run; so if it's possible, you may want to fill the tub while she's elsewhere, and just take her into the bathroom when the water is turned off.
And if she still doesn't like baths, then just wash her with a washcloth. She can get "clean enough" that way, without scaring the bejeebers out of her. :-)