I would start really slow with her... my new girl was neglected badly, and afraid as well - I just got her two weeks ago. My strategy was to keep her separated from the other horses, and keep her stalled at night. Her stall is where she gets her hay, grain, and supplements. During the day she is turned out in a paddock alongside the field my other two are in, but I leave the barn door open so she can go into her stall as she pleases.
Every day, several times a day, I go to the barn and mess around. Clean the stalls, sit and watch the horses, brush them down... just hang around. I kept treats on me the first week, and would patiently approach her (she didn't want me to get too close), give her a treat, then leave. After the first few days she would approach cautiously to get a goody. Then I started grooming her in her stall at night while she ate, at first only her neck and shoulders - now I groom her head to toe, including picking up her fronts as she eats... I tie her to pick up her back feet. She now also gets turned out with the others for a few hours, but is kept apart most of the time simply because she is on free feed to gain weight, and my other two are fatties :)
Start working on leading, lunging, walk and whoa - small steps to let her gain confidence. Rub her down with towels and blankets to begin to desensitize her. Lots of praise. Take your time and it will go fast... rush and she'll lose any trust you gain, and backwards you'll go.
After she begins to trust you is when to introduce the hose, and at first I would let her see you dragging the hose around, filling water tanks, etc... just no big deal to see the hose and water. Then I would hold her so she can circle you as you introduce a thin stream of water to her front feet, then move up those legs - just take your time, keep it matter of fact with no drama, and then leave it on a good note.
I am taking it really slow with my girl, and now, after only two weeks, my girl Ivy will come willingly to me, stand for brushing, allow blankets and fly masks, pick up all four and is beginning to lunge. She is also nearly completely white, and she is filthy... but we will do a bath slowly when she's ready.
Good luck with your new girl, it is very rewarding to watch them bloom!
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela