The connection between the pressure and the evaporation rate isn't so simple. Let's say that the water was held at a temperature of 99 °C, just below boiling at 1 atm. If you cut the air pressure (and hence the water pressure) by a factor of two, I think that puts the boiling point under 99 °C. So the water would boil, i.e. the evaporation rate (if you still call it that) would go up very drastically. On the other hand, if the water were at room temperature cutting the pressure to 1/2 atm wouldn't make a big difference.
Even when no boiling is involved, the evaporation rate is a tricky thing to describe, depending on the details of any surface contaminants of the water, how fast the vapor above the water is carried away by wind, how quickly the heat flows from the bulk of the water to the surface where the evaporation cools it, etc. What's easy to describe is the equilibrium vapor pressure of the water vapor over the liquid water as a function of temperature (look up Clausius-Clapeyron). Unfortunately, I don't know exactly how that depends on water pressure (which quickly comes to equal the air pressure), but it should be possible to look it up with a search.
(republished on 07/24/06)