Sometimes spraying water is vexing even for humans and let’s just imagine how vexatious it can be for an easily spooked animal. It is important to have the horse’s trust in you when it comes to washing him.
You should be able to prove that you are not a threat and the thing you hold in your hand is not going to do any harm to him. Washing the horse is not easy work as you have to do a lot of approaching and retreat. You have to be careful and you should really show the horse that you really care for him and his fears and that you are willing to do everything you can to gain his trust.