The average body is 60% water. In warm weather and with activity, we need more than usual. We lose water through urine, insensibly, and sensibly (perspiration), and in breathing.
I think the main thing is to stay a bit ahead with hydration - not crazy, but ahead. Some people tend to be unaware of thirst, some moreso. A handy way to know is urine color, or lack of urine.
Is coffee dehydrating? Nope.
Is alcohol dehydrating? Yes, so drink water too.