I would break this down much further. If your horse is afraid of walking on wet ground, I'd deal with that first. Could you not wet down a large area, and lead or lunge (at walk) this horse over that area, for quite some time. You could walk onto it, pause, and feed a treat, then walk off, rinse and repeat.
Eventually, the idea is that the horse would not worry about wet ground. Step one.
Step two, use a hose outdoors (if possible) to hose the horse, on a long lead line, and ALLOW the horse to keep moving, if he thinks he needs to. Start with bringing the water closer and then reatreating before he flees. work on getting it closer and closer. When he starts to try to walk away from it, you just keep the hose on his feet , neither increasing nor decreasing the water on her. You will have to have him circleing around you, and you will have to turn also, stepping over the hose as you go. It can be tricky.
When he stops circling, remove the water from his feet.
I'd work a lot on that. If the water is really cold, and the weather is not hot, then I'd not do this excercize too far up the leg. But, if the temp is not an issue, do it all the way up to his shoulders and back.
Step 3 is him being comfortable inside the confined space of the washrack. Others have talked on that. I guess you'd consider it about like training a horse to load in a trailer. Does this horse load well into a trailer/float?