For rides 20 miles and under I usually don't take any water. When I get home I'll mix a big glass of water or diluted sports drink and lots of ice. I have sometimes wondered if 20 miles is too much and I might do some experiments to see if a little water helps, but I'm pretty sure that for me anything 15 and under doesn't require bringing water. But then I frequently ride when it's really hot too.

Of course different people might have different thresholds so each person should set their own limits. You certainly don't want to get to a point where you are dehydrated, and yes by the time you feel it it's too late.