I'm confused about the relationship between air density, air pressure and gravity. Does water evaporate faster under low air pressure simply due to the air density being lower? If the air pressure was lower due to lower gravity rather than lower air density, how would that affect evaporation? I'm imagining an alien planet with 75% of earth's gravity, but the same air density.
Simple answer A phase diagram shows how the phase of water varies with pressure and temperature. Reduced pressure, regardless of cause, increases evaporation at the surface temperatures common on Earth (excluding those that form ice).
Complication On your low-gravity planet, each extra kilometre of altitude gives less density reduction (the length scale for an $e$-fold reduction is proportional to $1/g$) and more temperature reduction (this time proportional to $g$), so the atmosphere is colder and thicker than Earth's at a given above-surface altitude. These have opposing effects on how readily high air holds evaporated water. If we try to calculate the overall effect, we encounter further complications. For example, at reduced temperature the air thins more slowly, and moist air has a more complicated rule for how temperature falls at increasing altitude.