I have a senior pony that paws the ground and does not stand still for a bath. She mostly just takes steps from side to side.
My horse so freaks out with the hose blasting him that I've resorted to filling up a tub with water and using a bucket and sponge to apply it. He's not afraid of hoses or sprinklers but panics if he gets water on him.
I have worked with horses that are afraid of water and it is a real pain especially when you want to clean them before a show or cool them off on a hot day. You think they would enjoy it. I think the key is being firm, patient, and practicing daily.
Start this when you are not hauling somewhere and have lots of time. Go outside, roll out as much hose as possible and have a long lead on the horse. Begin approach/retreat with the water on -- spraying the feet first, then away, feet, then away. If the horse moves, follow with the hose until he stops. When he stops, shut off the spray. Horses learn just as much from when you stop your activity as when you start it. Keep doing this over and over, gradually moving to other parts of the body as he allows. Do this over several days, and you will be amazed at the results. The main problem is that people are in a hurry and want a quick fix. Take the time you need to address the problem when you aren't in a hurry!