One of my horses does the same thing. He won't stand still for baths and he snorts at the hose. My mom suggested it was the sound of the hose so what I have to do is to start by barely spraying any water and then slowly start spraying more and more. It'll take some time but it'll get them used to it. Hope that helped!
For a couple times, use a chain over their nose when bathing them. After that they stand still great. My horse was the same way.
Exposure helps the more around it they are the better they get used to it
The wash stalls at my barn don't have warm water unfortunately and she gets scared when the sponge drips so I have to not put a lot of water on the sponge
Maybe try using a sponge and gradually start using a hose. You have to get your horse used to cold water because not every barn has warm water:)
The horse I ride is awful for baths if the water is cold. She will stand nicely if the water is warm, though. Maybe see if she prefers warm water?
Honestly, I've had my horse for 5 years and she is the same way. I show so she gets bathed every week. Still hates it. So recently I put a hay bag up for her to nibble on while I bathe and SHE STANDS! Crazy but maybe give it a try!