Here are some things you can try, but if you have any doubts please find someone with experience to help you. You will need two people anyway. One person to handle the horse and one person hold the hose. Stand about 50 feet away with the horse and gradually try to lead it up to the person squirting the hose. If it starts to freak out stop and wait until the horse is calm before trying to go forward again.
If the horse refuses to go or backs up just keep asking him to move forward. Do not try and play tug o war because you will lose. What I do is stand at the side of the horse while pulling forward with the lead. If the horse is really stubborn I may try swinging the rope around behind me and tapping it on the side. If the horse backs up I just back up with it, but continue to keep pressure on the lead. (Always stay in the safety zone at the horses shoulder.) As soon as I get even a step forward I will release the pressure, and give praise.
Do not try to spray the horse until it can stand quietly next to the person squirting the hose. Once the horse can do this try running some water on him, I usually start at the shoulder. If he moves away just bring him back and try it again. The biggest key is patience. It may take several days or more for the horse to except this. Each time you can get the horse to except more than the time before you are making progress.
Often I will exercise the horse before trying to hose it for the fist time. This will help take some of the edge off. Remember to put safety first. Don't let you horse get between you and what it is afraid of. This way if he spooks he will be less likely to run you over. Also, I like to use a long rope instead of a regular lead. That way if the horse bolts I have some slack to give it. Working in a fenced in area is a must. That way if the horse does get away he won't go running down the street.