That 3 foot thingy is probably called "Golden Rod" - they make those a mile from my old house in Oxnard, CA.
http://www.drytheair.com/xcart/store/ca ... Watts.html
Note that the golden rod and other space heater like it are NOT de-humidifiers as the manufacturers claim. They are the opposite. With any standing water in the bilge they will actually result in more, not less, water vapor in the air. They are heaters. Coupled with a fan to force air changes in the bilge you may or may not improve the condensation and mold problem, depending on a number of variables including fan CFM, inside air temperature and %RH, outside air temperature and %RH and the thermal conductivity of the walls of the enclosure.
I sell a lot of monitoring equipment to building scientists, and the problem we are grappling with here is parallel to that of crawl spaces (and to a lesser extent attics) of houses. There is a long history of people applying many theories to crawl spaces and attics that sounded fine on paper but invariably ended in mold. The current best practice for both crawl spaces and attics is to seal and insulate them and condition their airspace the same as you would any other air space in your house. Unfortunately boats are not often plugged into AC power making power hungry solutions such as dehumidifiers a non-starter, and bilges of boats that are in the water are virtually impossible to keep dry between either deck leaks or below the water line leaks and the fact that bilge pumps often leave 1/2" or more of water in the bilge.
For those of you that are curious, I just placed two of my company's wireless humidity sensors on my 330 Express. One sensor is in open air at the helm station and the other is in the bilge on top of my auxiliary tank. They are logging temperature and %RH every 5 minutes. If you want to see the data go to our web site
https://www.omnisense.com/sensor_select ... larmOnly=N and login with username guest1 and password omnisense. You can see the current and historical data and graphs of the %RH in the open air and in the bilge. To see a graph click on the value you want to see a historical graph of. Note the sensors went in 10/31/2008 at about 9 am. This is step one in the process to quantify the problem we are trying to solve. Note also that there is standing fresh water in my bilge – best of my knowledge I do not have any below the water line leaks.
I recently replaced the o-rings on all my deck plates and schmeared them with Vaseline. At that time I had water and mold below the deck plates on top of the fuel tanks. Like a few others have noted here, I had seen water running into the bilge via my “sealed” deck plates when spraying them with a hose. I noted this morning that the area below my deck plate was dry and mold free. So could be the worst offender is leaking deck plates – a simple problem to solve.
Once I have accumulated a decent %RH data history for the bilge I’ll report back in.
Chris