According to the AAEP the ideal water temperature for maximum palatability is between 45-65°F. Horses tend to consume less water if it falls outside this range, making your horse prone to dehydration, poor digestion efficiency, and intestinal impactions.
Studies also show that horses tend to drink most of their water (as much as 82%) during the first 3 hours after feeding. You can take advantage of this by refilling their buckets with fresh, warm water at feed time.
When horses were offered water at 66°F they drank 40% more than when offered water at 32-38°F.