An important part of cooling out horses is evaporation. Keep a scraper in one hand while you hose down your horse with the other, and use that scraper quickly and continuously until your horse's temperature comes down.
If the water is not scraped off, it could act as an insulating layer (think blanket) and actually make your horse hotter than when you started. This is because the water absorbs your horse's heat and becomes warm.
Research prior to the 1996 Olympic equestrian events in Atlanta showed that a safe, efficient way to cool off a hot, sweating horse is to hose on cold (not warm) water and to quickly scrape it off. The cold water pulls the heat out of the horse's body, warming the temperature of the water now sitting on the horse, so it needs to be removed.