For the reasons that Rich pointed-out. e.g., You're just moving the problem deeper, you may be actually encouraging growth, if you got it hot enough to really sanitize, then the wood, fiberglass, wiring, etc. might be damaged. etc.

For me, if you can open-up the floorboards or cabinets, then I'd prefer to just clean-it out ( whether by a little mild chlorine solution and a brush/sponge -- or by Simple Green, etc.); and, then remove or reduce the moisture sources by ventilation. If you did want to "steam it", then you'd be sure it dries out and you can see what you're doing.

Blasting hot, saturated, pressurized vapors into closed areas is not the way I'd go.

If you can't open things up, then you might try and "ozone" like device -- though that has been reported to attack rubber and some wiring insulation if you use it excessively. Usually, a fan that sucks air through will work. IF you're in a marina, then a "Vornado" or other similar fan which will discharge air from the boat will work.

If you're just in an enviornment where you get FOG every morning and then it heats up during the day and cools off hard at night (say along the Pacific Coast of the US) -- then I would think you just have to either have positive ventilation or keep the boat "heated" during the evenings to avoid condensation. Since the water is pretty cold, I think you really need to work on the ventilation. I'd invest in a fan that blows air OUT of the boat; and therefore sucks air into the vessel.