Horses have no depth vision, so when you ask her to walk through water she feels like you are asking her to walk into an ocean with an immediate drop off. First of all, I really really recommend Parelli's Seven Games to build a better overall relationship. (http://www.parelli.com)
1 - Do not ever get frustrated. -Monty Roberts quote - act like you have 15 minutes, it'll take all day. act like you have all day, it'll take 15 minutes.
2 - first of all, try taking some one else along with an older horse that she knows. (preferably a horse up in the pecking order - if not possible, that's ok too) If she sees a different horse she knows/trusts doing it she may immediately get a lot better.
3 - walk in ahead of her or have a person walk ahead of you so she is assured she will not be instantly in water over her head.
4 - it may take lots of coaxing. the minute she comes in stop, and praise her. if she came in fairly willingly, go a little farther. if not, be done for a day.
5 - try taking her in when she's really hot and using a sponge to cool her down. then she'll learn it's fun and helps her cool down to go in. try feeding her a treat or too sometimes, but you want her to learn based on a bond with you, not just food. always praise her and do this lots.
hopefully she'll get over it!