"Jim Macklin" writes:

and kidney stones, which will ground you.

Kidney stones are not a universal consequence of even chronic
dehydration. Individuals who develop them often seem to have a
predisposition to the condition, or other problems that encourage the
development of stones.

People who have a history of renal calculi should drink more water and
fluids, but those who have no such history need not take any
exceptional precautions.

And simply not drinking much before a flight is not the same as
technical dehydration, which some pilots use to avoid urinating
entirely for long periods. If you're normally hydrated and not
thirsty, you don't need to drink anything before a flight. It's more
important to have something to drink on board. If you're the type who
can't tolerate anything in your bladder, a couple of urine collection
devices on board could be useful as well.

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