Vaughn Simon writes:

Dehydration gives no warning and can seriously degrade your pilot skills
just when you need them most.

Dehydration most certainly does give a warning: thirst. If you get
thirsty, drink. If you're not thirsty, you're not dehydrated. You
won't dehydrate enough in three hours to have any effect on your
flying ability unless it is extraordinarily hot inside the aircraft.
Make sure you are hydrated before the flight and you'll be fine.

On the other hand, having a desperate urge to pee and no place to do it can
also degrade your performance.

Much more so than the subclinical dehydration that might occur on a
short flight of three hours.

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