Why is it that devices to separate water from fuel are quite
common cars (my F-350 truck has one), but such a device, which
could save lives in an airplane, is not offered for airplanes ?
Even as a retrofit ?

To this and other similar questions, the general answer is weight, cost,
and reliability.

Weight - all the gadgets and gizmos weigh something, and this gets
removed from your useful load. If all the safety features people wanted
were included, the airplane would never fly. I guess that would be
pretty safe. Cars don't have to fly, so they can be as heavy as it
takes - a few pounds here and there don't really make a difference. But
in the air they do.

Cost - mainly certification cost, since any newfangled gizmo has to go
through the FAA wringer to ensure that there are no unexpected
surprises. Whether this is actually effective or not is subject to
debate (witness the homebuilt arena) but it is necessary.

Reliability - it's another gizmo to go TU, with unpleasant side effects.
Cars can pull over when they break. Planes can't. In the case of
this particular item (fuel/water separators), it would still not relieve
the pilot of the necessity to check for water in the gas, since we all
know that things that "shouldn't" happen, do anyway.

Jose
--
Money: what you need when you run out of brains.
for Email, make the obvious change in the address.