I did not need one on the Pacific Coast, nor the NT. I did beg water once from an RV on a hot ascent of Washington Pass in the Cascades.
Fred, I was just about to post that a water filter would be a good thing to have climbing Rainy and Washington Passes!
The alternative is to take 2 2 liter Platypus bladders, and fill them before you start the climb. One nice thing about the ACA maps is that they'll give you notice when there's a long stretch without services. That's a clue you need to fill the bladders before you hit that stretch. Otherwise, they fold up small and light in your panniers.