Careful using an electric motor near an open gas tank. The gas fumes will be pretty flammable and the sparks from the motor can ignite the tank. With hardly any gas in the tank, the mixture may be just about right for an explosion! If you decide to heat the tank, leave the cap on and heat from the outside, that will avoid the explosion potential. (ok, BC hat off)
It's really difficult to remove 100 percent of the water without taking the tank off. You will have a much easier time getting it all, then have a LOT better assurance that when you are on the trail that water won't be sucked into the fuel pump/carbs, stranding you alongside the trail.