The best gauge for hydration is the color of your urine. Pale yellow/clear is ideal. You will need some electrolytes in your fluids as too much water can lead to
water intoxication.
No need to go crazy, a couple of bottles of a sport drink will be fine. If you eat a normally salty American diet, you probably won't even need that.
I only use a sport drink on my weekly long ride.(I do love the salt.
)
There are low calorie sport drinks if you're worried about the sugar content.
Warm/hot water is not harmful but ice water will absorb excess body heat. And you won't cramp.
Freeze water in your bottle(part full) and it should last the whole way home.
Drink in small sips during the day. Too much at one time will cause the body to overcompensate with increased urine production.