You are correct, though at some point we figured out how to carry it, stay close to it, and establish access.

I've been told by a few wise people to wait to drink until I'm done trekking, by the way.

I don't think we should blindly do anything, and abandonining millions of years of evolution and wisdom especially.

> I've been told by a few wise people to wait to drink until I'm done trekking, by the way.

I'm not sure what distance we are talking about but doing any kind of sport (even light exercise) you should start to drink after at most 1 hour.

If drinking wasn't necessary, long distance runners or athletes of other endurance sports wouldn't drink. You need to stay hydrated or you'll face fatigue, muscle problems and generally perform worse.

Trained athletes will be able to drink much less but that is because their bodies are more efficient, they still need to hydrate for maximum performance. The 1 hour rule isn't set in stone and you can go 3-4 hours without drinking but it has many drawbacks.

Hydration actually isn't very important for endurance athletes in events up to a couple hours. The real benefit comes from carbohydrate supplementation. The water just acts as a convenient delivery channel to get the carbs into the athlete's body quickly.

I’m quite certain that is strongly dependent on the ambient temperature.

I'm talking about a several-hour moderate difficulty hike without running or high-intensity exercise, which you started in a state of adequate hydration.

I've tried out several different approaches many times, and the best I've learned is to wait to drink when resting, no sipping in between.

I am not a doctor and this is not medical advice. YMMV.