We just got a tankless water heater. There is something about body temperature water; my horses are totally calm with a bath now.
I noticed that when I give a horse its first bath, the force of the water is important. If your hose water is really cold right now, just start at the feet and legs. Don't tie. Hold the halter lead and let the horse move away from the water while you splash it on his feet and legs. He will feel less threatened if he can move away. Follow as far as you can, remove the water and coax him back to the starting point. Try again. I think tying is the worst thing you can do.
Same with a sponge and bucket of water. Don't tie. All that jerking makes him panic and it's really hard on the poll and neck.