Start with sponging her legs with water to get her used to it then work your way up. Once she's used to that try the hose ok her legs. Take your time and make sure you let her know when she is being good(treats work great)
Start with a hose very far away. If she stands still, reward her. Bring it closer step by step, rewarding when she stands still. Once she tolerates it near her start holding it/ picking it up etc, then I like what other people said, touch her with it, start having the water running a bit etc.
The reward can be whatever she likes... pats, ear rubs, *small* treat used occasionally if she is particularly good...
Spray them on the legs (while someone is holding them.... not tied) and wait till they stop moving. Once they stop moving (right when they stop) take the water away to release the pressure, and then repeat until they grasp that if they stop moving they will be rewarded.
I would say leave it out so your horse gets used to it. And then slowly introduce them to the water. Do a little at a time
I would try taking part of and old hose and rubbing it on him/her while they are doing something like eating.