I've mentioned dehydration numerous times over the years in trip reports.
I too drink from the lakes when safe. I found an old 16oz aluminum cup, the kind with the wire handle on a portage one year. It's clipped to my daypack/ditch bag that sits right in front of me.
The simplest way to see if you staying PROPERLY hydrated is to check the color of your P. This works for everyone unless you're taking a drug that would cause a color change?
You want to P as CLEAR as you can, almost like clean water. If you're NOT, you need to drink until you DO P clear. They have a color chart over the men's urinal at the Quetico Park office in Atikokan.
Many people make the mistake of thinking because they drank X amount of water that is enough?
What they don't realize is that you DON'T feel thirsty UNTIL you're actually dehydrated.
So the water your drinking is REPLACING the fluid you've lost BUT you still need more to keep you from BECOMING de-hydrated as you keep sweating it out.
That fuzzy thinking, body aches/fever is your body trying to tell you it needs fluid.
The goal should be to NEVER get to the point of that happening.