If it IS mold,
you might get relief from a heavy dose of O2. You can rent a commercial ozone generator like those used in hotels to get rid of tobacco smoke odors. Ozone WILL kill mold, but you don't want to be in the boat while you're "ozoning" it, and be advised that it will tarnish brass and accelerate rust, and isn't good for rubber products either. That being said, ozone has the capacity to get everywhere on your boat, and for a once-in-a-while solution it might be worth investigating.