It may take a lot of time, but the best thing to do is expose your horse to water often. Eventually she shoukd get used to it. Use the hose, go out in the rain, go by ponds and rivers, etc etc. Once your horse realizes it's not a danger, she should start getting better.
My horse also had that tremendous fear, to help him I put him on his halter and lead his towards the water let him see/smell it. Then I entered into the water to show him there was nothing to be scared of, and soon enogh she will go in. If that does not work circle her around the water and gradually make her go closer. Or do both to give her confidence