There are several reasons that can explain why your horse doesn´t like to drink water:

Some horses will not drink water simply because they do not like or are not used to the taste of water. If you have one of these guys, find a flavor such as Gatorade or apple cider vinegar and add it to the water to encourage drinking.
Some horses do not drink well because of internal dampness, which is associated with the spleen and is characterized by heaviness, stickiness and edema. If your horse has a tendency to stock up in his legs, have puffiness under the eyes, swelling or thick coating on his tongue or susceptibility to moist skin eruptions, he may have internal dampness.
Some horses prefer drinking from a trough over a bucket or automatic waterer.
If your horse has sore teeth he will back off on water drinking especially if the water is cold.
If you can't get your horse to drink, cut out his grain and cut back on his hay until you solve the problem. Offer fresh grass, bran mashes and moisture rich treats such as carrots, apples or watermelon.