Caroline,
I would advise against leaving the water bowl empty. Unlike other smokers, it is not designed to introduce moisture to keep things moist. Although it will introduce some, its primary functions are to collect fat drippings & extinguish the spent pucks.
Without water, the collected fat will not only not extinguish the pucks, but possibly ignite them. Best case scenario, you get that acrid burnt flavor in your food. Worst case scenario, the smoker superheats & becomes an inferno as soon as you open the door & give it oxygen. I've got pictures if you want to see them. (No, it wasn't mine.)
You're on the right track with leaving the vent open.
Kirk
http://www.chezbubba.com Ya think next time I check into a hotel & they ask "Smoking or Non?" they would mind?