I have to agree with not tying them, when I got my horse Romeo he came in with a mile long list of problems (mostly behavioral) one of which was HATING baths (he would try to bolt, flip over and kick the water/me), we have a wash stall with cold and hot water and cross ties, he hated being in there, and would attempt to flip over when i would turn the water one, so aside from not being able to tie him (when tied he would try to hurt himself) i couldn't bathe him either... so what I did was (it took about a 10 times)
had another person with me, and had them hold on to him with a lunge line ( a regular lead is too short and wont work) i gently talked to him while while i SLOWLY ran the hose near and across him, eventually after he became ok with it, i would do it (on another day) with the water on but on touching him... each time making baby steps... eventually you will be able spray him off, horses always move away from things they dont like or are scared of, so if you spray the water above the (like a shower) they usually will not rear,(mind you this is after at least a wk or 2 of conditioning to the hose and sound of water) and the lunge rope is in use to allow her to move around, however when SHE stops movement you STOP with the hose... she will learn to associate good behavior (i.e. not moving) with the water stopping.

OH and any FYI Romeo (my much loved psycho horse) now loves baths and is ok being tied (so there is HOPE ) however, him i use cold or cool water on him he prances around and freaks out, he LOVES steaming hot bathes, so maybe in conjunction with other things water temp is an issue also?

just a thought
Karen