You just have to practice getting her in water where ever you can. The more times you tell her to get into the water and she does, and realizes she's survived, she'll get more confident. Be careful, however, because if there comes a time where you come across water that is particularly boggy or whatever, and she has a bad experience she will take it as you lied to her about her safety and her confidence will deflate. So make good judgement calls.
A good thing to teach horses to get into water is this. Keep their head at the water. Ask for her to move her butt to the right, then to the left, and keep repeating. Don't ask for forwards. Just keep telling that hind end to move. Any effort on her part to -look- at the water, or attempt to go forwards, praise her big time. Don't let her turn away from the water. If she backs up, doesn't matter just keep moving her butt from right to left. She will on her own decide forward is a more comfortable way out, and then you praise. You keep repeating until she wants to take -you- into the water. With timing and patience, this method never fails. You just have to be clear and consistent, and most of all patient. Don't ask for forward, you will only make her more resistant. Get the forwards by her deciding its easier.
In riding, a horse's energy is like a river- guided by the banks but not stopped by them.