The entire health of a boat is mirrored by the health of the bilge area. We are not talking spotless here, we are just pointing out basic safety and health requirements. So what's the cause?

Since it is hard to stop this in-flow to the bilge, it should be reduced and managed.

The issue of mold and sanitation is our first concern.

Mold

Mold forms as a living feeding creature. It needs a food source for growth and cool damp dark areas are it's favorite lodging. The best way to reduce mold and mildew is by venting the bilge area as best as possible and keeping the bilge area clean. There are also products like "Mildew Gas" which act like big moth balls to counteract growth. By the way, sunlight is the natural enemy to mold/mildew, so let the sun shine in ...

Constant air movement is the best agent to fight mold. Whether you install a power vent, de-humidifier, or heat stick, make sure it operates on a regular basis.  Pricing of "Solar-electric vents" is becoming relatively reasonable. You should shop around and install at least one in the cabin top of your boat to encourage air movement.

Since moisture is the catalyst, it is well worth getting down to solving those annoying unknown leaks. General Sources:

• One system often overlooked is the shower sump. Most shower sumps serve the shower and the floor of the head area. Even if you don't use the shower, periodic wash down of the head area means this system should be working properly. Some shower sumps aren't sealed with a lid to prevent a potential overflow. It wouldn't be a bad idea to try to fit a lid. If that isn't possible you need to periodically test that the discharge pump and auto-switch are functioning.
• Air-Conditioning, refrigerator, and icemaker condensate lines are a bit tough to resolve but, if they can be run to the shower sump or over the side (thru-hull) you'll find you won't have as much dampness in the cabin area and under your decks in the bilge areas.

Sanitation Systems

Sanitation leaks aren't always because of the system itself. Sail boaters often have sanitation leaks because of the nature of using the facilities while under way. This spillage then is often washed into the bilge area, thru the floor drain of the head area. Why sail boat builders don't put shower sumps into sail boats, I'll never know... Anyway, sanitation leaks are easy to find. It's rarely a fun job but the rewards are substantial.

If your sanitation system is older than 10 to 15 years old, you probably need to look closely at the end fittings and the hose itself. The older plastic end fittings have a tendency to become brittle and crack. The hose doesn't necessarily start to leak a much as it starts to smell from absorption. If you note discoloration or crystals forming near the fittings, it's definitely time to start replacing with new.

Engine Sources

Besides being against the law, engine oil leaks and spills need to be reduced and controlled.

Actually, it's not against the law to have an oil leak but it is against the law to pump bilge water that contains oil, over the side into the water. It is the boat owners obligation to prevent oil discharge of any sort and amount.

The best way to reduce unwanted oil emissions into the bilge is to maintain your flame arrestor, replace any PCV valve on the motor, and to use extreme care when changing your oil and filter.

Sounds too easy? Those are the biggest causes of oil in the bilge. The next cause is oil leaks due to mechanical failures.

If you've been inspected, lately, you know that they will look for an oil absorption sock and/or pad under each motor and/or the lowest part of the boat. Although they may not be black with oil and dirt, it is still a good idea to replace wet or water-logged ones every couple of months.

Green anti-freeze is toxic and every attempt needs to be made to prevent it's discharge. Besides loss during anti-freeze changes, the one biggest cause of pollution by Green anti-freeze is people who use it to winterize their engines. It is against the law. Don't do it.

Commercial Bilge Cleaners

There has been a bit written lately about bilge cleaners and their affect on the environment.

Even though it may say "Biodegradable" on the bottle, the concern is more about what the cleaner is dissolving during the cleaning process. It is this solution that is pumped over the side that have people concerned. If your just doing a quick freshening of the bilge to reduce mold buildup once a year, there shouldn't be much of a problem. But if your constantly doing it to clean away the evidence of oil and sanitation  leaks, you need to look at fixing those leaks.

Clorox Bleach as a Bilge Sanitation Device

Although not recommended for sanitation systems because of the damage it could do to hoses and fittings, a 10 to 1 mix of Clorox Bleach or equivalent, put into the bilge water on a weekly basis can be effective in reducing mold. Usually within 24 to 48  hours, the bleach slowly evaporates and acts similar to the mildew gas produc