Above the mesophere, the upper atmosphere has negligible water vapor
content. The Earth's atmosphere contains a so-called "cold trap" at
about 75 km, where the temperature drops to about -100 C. This freezes
the rising water vapor out of the atmosphere.

Good thing, too-- it's why we're here. It prevents water vapor from
rising to the top of the atmosphere, photodissociating from solar UV,
and leaving.

In any case, water is a trivial weight fraction of the atmosphere-- the
drag you'd get from scooping atmosphere would far far exceed the
delta-V you'd be able to get out of electrolyzing water.

--
Geoffrey A. Landis
http://www.sff.net/people/geoffrey.landis