Abstract: A weather cover for a boat comprises a fabric cover that completely covers the boat while floating in the water. The cover is gathered tight above the waterline. An air-duct fan inside the cover draws in clean dry air from outside through a filtered vent and a connecting flexible air duct. Such fan runs on electricity provided by shore power, the boats electrical system, or solar panels. The exhaust air is directed through another vent through the cover. The intake cover vent is filled with a replaceable filter foam material. In both vents, a louver opening is directed downward to prevent rain or water spray from entering. The fan need only keep the air inside the boat dry over the long term. The fan therefore may run continuously, or as power is available, e.g., during daylight when the sun is shining.