Water is good, but you have to replace the electrolytes you are losing through your sweat (that gritty salt thing.) I will usually bring some packets of Gatorade powder to add to my bottles at refill stations, when I can find them.
Heat exhaustion is no laughing matter. I keep an eye on my heart rate with a HRM and display on my Garmin. If I notice my recovery time after a short effort starts to get too long (and 6 hours post ride is WAY too long) I know it's time to back it down, maybe even find a shady spot to sit and drink a bottle.