How to Get Your Horse Accustomed to Baths and Water
Pick the Right Day
I've often found that the best time to introduce a horse to a bath is on a day when you wouldn't mind a cold shower yourself...a hot day in the middle of summer.
Never introduce a horse to bathing in winter - you'll set it up to hate it forever that way. Although horses feel the cold less (and the heat more) than we do, nobody wants a cold shower on a cold day.
Have the Hose Around
Introducing your horse to hoses can and should be done early...ideally with a foal or a yearling. Many horses find hoses a little disturbing, perhaps because they resemble snakes. Even a trained horse may spook at a hose that's moving if they can't see the person dragging it. It's perfectly natural for your horse not to like things that slither around.
At some point, it's a good idea to let the horse inspect a hose thoroughly - ideally by hanging it on the paddock fence while the horse is turned out and letting them deal with it in their own time. (This is a good approach with any scary object that the horse is unlikely to do significant damage to. If you do it with a saddle, pick an old one that you don't care about that much).
However, it's best not to use a hose the first time you bathe your horse, even if the horse is used to them.
Start with a Sponge Bath
The best "first bath" to give your young horse is a sponge bath. Start on the back, hindquarters, and neck - these are all less sensitive areas on which the shock of cold water will be less intense. Work up to bathing the horse all over. With some horses this may all happen on one day - with others it might take as long as a few weeks. (I've even known horses whom I've just been able to stick in the wash stall and hose down despite never having been bathed before. You should know your horse and be aware of what they're willing to do).
As another note, if you're in a hurry and don't have time to bathe your horse on a hot day, sponging water onto their pulse points will cool them down quickly - under the tail, in the "armpits" and, if they'll let you, the base of the ears.
Introducing Hosing Down
The quickest way to cool your horse off on a hot day and the starting point for a pre-show bath is to hose the horse down.
Some rare individuals will take to this right away - again, you should know your horse. I've also known particularly nervous horses who never really learn to tolerate it.
Do not hose your horse down for the first time on your own. Always make sure somebody else is around, and ideally have the horse held rather than tying them - it's generally safer. If your horse decides it's having none of it, breaks free, and runs off, it's always helpful to have somebody to catch them. You may want to keep them in some kind of enclosed area and I do not recommend cross-tying in a wash stall for the first time a horse is ever bathed - you're likely to end up with something broken, like your cross ties, the stall mats, you, your horse...
Start by just turning the hose on and pointing it at the ground next to or under the horse. If he's fine with that, move on to the large muscle areas - back, neck, hindquarters, then the legs. I generally do not hose down a horse's head. If the horse wants to drink from the hose, however, let him. Some horses like to do this...although keep an eye on them. I have actually had a mischievous pony grab the end of the hose and turn it on me before now! (Fortunately it was a hot day).
One final tip. When introducing anything new to your horse, always allow plenty of time. I've seen people get into all kinds of trouble this way. Your horse's first bath should not be the day of their first show...and yes, people do this. You'll end up late, frustrated, and mad with your horse. Instead, do it when you are in no hurry and choose a time you and your horse are relaxed. Be prepared to spend all of your stable time on it if necessary - and your horse will appreciate it. Most horses learn to appreciate a "cold shower" after riding on a hot day.