i wonder if maybe you start her small and then work up to the bath. What about letting her sit (in the bathroom if she will, or another room if it's the bathroom that's scaring her) with her feet in a washing up bowl of warm water and give her flannel wash with the water, then slowly build up - next step, the garden plastic bin sized thing, as mentioned further up the thread, but still just letting her stand in it, or sit with her feet in it. Just gradually build her up to accepting being in a large space with water. I have also seen people putting a washing basket inside the bath. The water can get to her, but she's in a smaller space, which may give her confidence.
Dolly needing a bath/shower is also a good suggestion.
Whatever it is that has spooked her, I think the suggestion further up of forcing her into the bath is horrific. You can't cure a fear by making her more frightened.