I will be leaving primary residence for 2-3 month period. When I leave (for the winter months), I shut the water coming into the house, and drain water from lines. The water is still fed to the boiler for heating the house, The hot water heater is a 75 gallon Bradford White, the tank will be full when I shut the water to the house off. Can I leave the Pilot light on all the time, is there a possibility of too much water evaporating from the heat produced by the pilot? I can shut the pilot and gas supply to the water heater, it is a bit of a hasssle to re-light the pilot, but will do it if it is saffer. I would prefer to just open the water supply to the house and turn on the water heater control button to the on position when I return home.

You can do it either way. I'd leave the pilot lit. It will cost you a couple dollars but the lit pilot should keep the moisture out of the heating chamber and reduce the rust and corrosion.