Some schools alter their policies for wrestlers a few days prior to the urine specific gravity test allowing them to carry water bottles in school. While this helps them hydrate for the pre-season test it does not help with their hydration needs during the entire season. While water is never a bad choice sometimes it's not the best fluid for the job. Water is a thirst quencher. It turns off the thirst mechanism and athletes stop drinking before they're fully rehydrated.
Water is also a natural diuretic.
It signals the kidneys turn on and produce urine. They contain small amounts of sodium and glucose. This combination has been proven to promote re-hydration. Sodium turns on the thirst mechanism and athletes are more likely to keep on drinking. The small amount in a sport drink will promote re-hydration but not excess fluid retention.