This is really important in hot weather. Even more important is to make sure to actually drink it, even if you don't "feel thirsty." I live in the PNW, but on the east side of the Cascade Mountains. Last summer it averaged maybe 95-100F where I live (central WA), and I often rode my bike during peak temperatures (1pm-4pm or so).

I did a lot of solo riding between April and June in preparation for a Seattle to Chicago ride with some family members who live on the west side of the mountains right near the Puget Sound where it is significantly cooler. First day riding with them we went about 100 miles from Anacortes to Lake Diablo (western foothills of the mountains) and everything went great. All three other riders mentioned how much water I drank and how I was always "glistening with sweat," which is completely true because 1) I naturally sweat a lot (even when I'm cold) and 2) drink a lot of water when I ride. The next few days of riding went a little differently:

As we descended the eastern side of the mountains and made our way through "my part of the state," temperatures rose significantly and we were putting in long days in high heat. On the second day, all three of my riding partners started experiencing some complications due to heat and dehydration. Of course they weren't as acclimated to the heat as much as I was, but I think the bigger factor was really that they just weren't used to putting as much water through their bodies as I was. I was carrying two 25oz. bottles on my frame and a third 21oz. bottle in my jersey, and ensuring that I was making my way through at least 50oz. every 25-30 miles. Hot weather sucks, but not drinking enough water is definitely worse.

If you currently only drink when you feel thirsty, I suggest incorporating a drinking schedule into your training rides to avoid dehydrating.