Cold water can cause stomach upset in some people, so cool water is best.
Drinking something lukewarm may not be as refreshing to most exercisers. Do what is most comfortable for you. For exercise less than 1 hour, water is best. For exercise greater than one hour, a beverage containing carbohydrate and sodium is best for optimizing gastric emptying and intestinal absorption. Flavored beverages increases your thirst which helps exercisers want to drink more.