You mean the water molecules in the air?
Humid air has less oxygen per unit of volume (i.e. less dense) available for combustion, hence weaker mixture and lower engine power output. Air at sea level is denser than that at higher elevations or in the case of a/c, flight altitudes. That is the reason to require a pressurized cabin above 12,000 ft--there is insufficient oxygen in the less dense air to sustain rational mental function... p.s. Good catch on finding the leaking fuel line.