Although you've probably heard otherwise, research has shown that it's safe to let a hot horse drink cold water after a hard workout. In fact, hydration in the warm weather is so important that you may want to consider using incentives to get your horse to drink more.
One easy method is to flavor the water. Mixing in apple juice or some other flavor can make your horse's water more appealing. This is particularly helpful on the road because it masks the unfamiliar taste of "foreign" water.
A favorite trick of endurance riders is to feed watermelon, which is 92 percent water. A horse will still need water, but ingesting watermelon can help in a pinch.
One last option for encouraging drinking is providing your horse slightly salted water immediately after exercise, which will stimulate a stronger thirst response. To do this, simply mix one tablespoon of salt per gallon of water and offer that mixture to you horse. Then, 20 minutes later, replace the salted water with plain water. Research has shown this method will trigger thirst, and the horse is likely to drink more of the plain water than he would have otherwise.