[onewhippedpuppy]

You guys are overthinking this. A bilge blower, or multiple bilge blowers, are exactly what he is asking for. Available in multiple airflows and diameters, designed to exhaust air out of a bilge, designed for a marine environment, designed to be attached to a flexible duct on either end, available in 12 VDC for easy connection to the boat electrical system (also available in AC), and cheap. All of my boats have had them, I think the current one has one that is connected to a 3" flexible duct (think dryer vent) to suck flammable vapors out of the engine bay prior to starting. I'd be willing to bet that a household exhaust fan wouldn't last very long in a damp marine environment.



If you keep the boat sealed up, you could also add a desiccant designed for a marine environment to your dehumidifier. I've always taken the approach of trying to promote airflow, but my boats are probably smaller too. For winter storage we have a top with vents and take everything out of the boat. We also keep it in dry storage so that helps too.