When you stow the boat next winter you can help keep mold at bay by leaving a bunch of eucalyptus aboard. We have 2 of those vents and those combined with the eucalyptus really did the trick when we used to store the boat.

Bill

I believe that the Chlorine in bleach is the one that is enviromentally un-friendly.

If you can use other type of mold destroyer, it would be better for everyone.

Read more at places like Wikipedia on bleach and its environmental impact.

http://en.wikipedia.org/wiki/Bleach

I live in a northern rain forest in Sitka, Alaska. Average january mean temp is 33 F. 90 + inches of rain a year. I keep my boat in the water over the winter. Needless to say, humidity is an issue. I have found these things helpful to keep mold that thrives on the moisture under control.
1. Bleach. I wipe down all of the walls several times during the summer and once in the fall before I leave with a 10% solution. Chlorine is the active ingredient and it is in your drinking water, but it is toxic and does gas out of the water into the air. I keep the boat well ventilated until I am done
2. Calcium Chloride crystals (Dry Z air). I put 5 baskets aboard. My boat watch person changes them 3 times during the winter. In the summer, I use Damp Rid hangers in the closets.
3. During the summer cruising season, I keep my clothing and most linens in giant zip lock bags to prevent them from absorbing moisture and molding.
4. I keep the engine heat on when motoring to dry the boat out as much as possible.
5. In the winter i put in two electric heaters with fans on timers set to come on for 2 hours twice a day. They bring the temperature up to the dew point. The moisture in the air will then condense on the calcium chloride crystals.
6. For the winter I remove all mattresses, clothing, books, paper, etc and put them in a heated storage locker. I pull the seat cushions away from the walls and keep the cabinets, storage areas, bilge covers, refrigerator open so air can circulate.

I have done this for the last 13 years with good results. When I first bought the boat, it was in Seward, Alaska. There the winter temperatures were colder. I tried without the heaters and came back to a boat with frost on everything...just like the scene in Dr Zhivago...
Lesson learned.