My horse used to be so bad for this. I got a wet sponge and squeezed it out on her and got her whole body wet and than I turned the hose on lightly and let her check it out and than I put it on her front leg and when she stoped moving away from the water I took it of of her leg and if you slowly introduce the hose like that to her whole body than she should get used to it.
Whatever you do don't start in an enclosed area where the horse will feel trapped. Start with a little trickle from the hose and start on their legs. Hold it there until the horse relaxes then take the pressure away. Continue this routine slowly as you work your way up their whole body. Remember that the first time should just be a good experience.
Start off in an enclosed wash bay. Have someone holding while you start the hose gently on the front hooves, working your way up as horse relaxes with it. Plenty of praise for good behaviour & I use verbal reprimands on my yearlings when they are naughty. Don't stand back & spray, just let it run down leg so they don't accidently get sprayed in the face & get a fright. Rubbing where the water is landing helps ease the ticklish sensation as you get up onto the body