There have been a couple of recent posts on dehydration / heat exhaustion in the Gear section, pfd topic. I thought it would be worth posting a reminder here just in case others might have missed it.

One poster suggested that overheating is "worse than a bad cold". It's more than that: dehydration and heat stroke can be life-threatening if left untreated.

Two of the main things to look out for when canoeing (or doing any other kind of exercise) in hot weather include lack of perspiration and yellow or gold urine (as opposed to nearly clear). If you're not urinating at all despite drinking a lot of water, you're not drinking enough.

A lot gets mentioned about electrolytes and the need to replenish. This is not as big a deal as it's made out to be: we all get enough salt in our diet as it is. Electrolytes do make it easier to drink more water and for the water to get absorbed into your bloodstream, but if you didn't bring Gatorade, Tang, or whatever, just keep forcing the water down, whether you feel thirsty or not.

fwiw, a couple of weeks ago on a 36 C day (not very strenuous canoeing or portaging), I put down over 10 litres of water. So don't underestimate what's required.