Moiature in the fuel doesn't affect performance much. One of the writers, for Model Aviation if I remember correctly, added as much as 10% water to his fuel to test the "moisture" theory, and he actually found a small increase in performance (RPM) with a little water.

It wasn't a real scientific study with tight controls, but it convinced me that I don't need to worry about a little water in my fuel.

A little fuel left in the engine however, will attract moisture and cause rust and corrosion.
Dennis-