The first problem with this plot device is that it’s pretty dumb. Hydrogen and oxygen are two of the most common elements in the universe. An alien race is savvy enough to master interstellar travel but too clueless to combine two Hs with one O to form H2O? C’mon.
Something we have not been able to do as yet I think,
but then we haven’t mastered interplanetary travel
yet either.
Hydrogen - the most abundant atom in the universe...
must be the oxygen that’s rare.
Actually, we mastered it ages ago. Take two parts H and one part O by volume and light a match.
Space shuttle fuel cells did it all the time. Water is the output. On the Apollo missions, the astronauts complained the mix was H2 rich, and gave them very smelly flatulance.
/johnny