Me: Drink some water.
Bob: I'm not thirsty.
Me: If you're thirsty, you're already not drinking enough. I'm not picking you up off the sidewalk.
Bob: *sigh* Okay.
Looking for a little ammunition for the next water battle, I did some research. There are some medical reasons why you could dehydrate quickly but unless you are experiencing any of those it would be ridiculous to dehydrate just because you didn't drink enough water.
And why should we worry about dehydration? It can result in confusion, weakness, dizziness, fainting, vomiting--I could go on. What a way to ruin a travel adventure, right?
Get truly concerned however if you or your companion is running a fever, fainting, confused, has a headache or seizures, difficultly breathing. That's when it's time to seek medical treatment. Best to avoid all of that and just be sure to drink enough water. How much?
You've heard of the 8X8 rule: eight eight ounce glasses of water per day. Actually you need a little more than that but it's still a good rule and any fluids can be counted toward that. Just remember that all fluids are not created equal. Caffeinated drinks like coffee will help to drain fluids from your body and alcohol is not your friend on a hot day. It will also drain fluids and impair judgement if you are near dehydration.
There are other ways to get fluids besides drinking water. You can eat watermelon or spinach which contains a lot of fluids. Suck on some popsicles or frozen sports drinks you could make yourself. Still, the best source for your fluid intake is good old water. No calories. And thanks to the novel idea of bottled water, you can usually find good safe drinking water anywhere these days.
Don't ruin your trip. Hydrate. Drink up. Stay healthy.