john smith writes:

Which leads to serious dehydration and potential mental impairment.

No, it does not.

You can be minimally hydrated and produce only the minimum of urine
required to remove waste products, and this can allow you to go for
many hours without urinating. It's a fine line to tread, but it's not
unhealthy or dangerous. Personal experience can teach you how much
you should or shouldn't drink in the hours preceding a trip in order
to find a good balance between necessary urine production and
overproduction that might make a stop necessary (or worse).

Dehydration occurs from losing water, which can be prevented by
bringing suitable beverages along in the cockpit. If you feel
thirsty, sip something to drink (make sure it's not diuretic, which
rules out Coke, tea, coffee, etc.). Unless the cockpit is very warm
and you are losing a great deal through perspiration, you'll be fine,
at least for the duration of any GA trip you might take. By the time
you'd have to worry about fluid balance, you'll be out of fuel,
anyway, so there isn't any danger.

If you will be flying in conditions of heat (effective or real),
consider installing air conditioning. Apart from preventing excessive
dehydration, it will make you more comfortable and better able to
concentrate on your flying.

Having a full bladder is a strong distraction and can be potentially
dangerous. Taking an occasional sip of a drink if and when you feel
thirsty is much less distracting and more than adequate to ensure
proper hydration. Making provisions to urinate in flight is also very
potentially distracting.

--
Transpose mxsmanic and gmail to reach me by e-mail.