One of the bigest things I learned with my crazy boy, is start on a longer training lead, outside. Do not start in the wash rack! This doesn't allow the horse to learn to accept the "pressure" of the water by himself, but it is forced upon him, making his fear worse. Before you start with the water, make sure he/she knows how to yield to his hindquarters. This is useful for when they try to "run away" from the water. Once they are still, try spraying away from them by their side so they can see the water. Next, work your way up to spraying the bottoms of his legs. Then the whole leg. Next, the chest. Eventually start working your way towards their side/flank area (making sure to stay out of the way of their feet). Do this with both sides.
This is a process, so it takes time and you have to stay consistent. Give breaks between each section so they don't get overwhelmed. Reward, reward, and reward. This is a big thing for them!!!
Once they are completely comfortable being hosed off in hand, start the same process again indoors, or the location you will end up bathing. Good luck!!! Mine, as a 3 year old, hadn't ever seen water. One time he even pulled a post out of the gound because the hose was on about 5 feet away. 2 days after working with him, I could spray him pretty much anywhere. It might not be that fast, but don't focus on the time it takes, but making the experience positive for your horse. Hope this helps! Good luck:)
If the horse is afraid, he must have had a bad experience.
Use first a sponge drenched of water, then a brush, then the shower's spray. When he does not move, congratulates him, hug him...
Be very patient, and remember some horses (or animals like cats) do not like water at all, you cannot really force them to appreciate water.
Try the positive reinforcement method. As soon as the horse gets closer to the water or the spray of the shower, congratulates him and/or give him a treat.
You can do the opposite: with the spray of the shower, get closer to the horse and as soon as he does not move, congratulate a lot!
You have to be very patient but if you keep doing this method, your horse will associate the bathing to treats and cuddles :)