There's not many bilge's that can boast that they've been oil free. Every bilge has had an oil or fuel spill at one time or another. So before pumping out the bilge, check to make sure it's oil free.
Cleaning oil and fuel out of the bilge is really quite simple. Add a generous amount of dish washing detergent (Joy works great) to breakup the oil. Mix the detergent thoroughly throughout the bilge. With absorbent paper towels or sponge, absorb the oil residue floating on the surface and setting on the bottom. Put the oil residue in a plastic bag lined with newspapers and deposit it at a proper oil disposal site.
To prevent spills into the bilge when you're replacing your oil of fuel filters, line the area with newspapers or paper towels, then wrap a plastic bag around the filter before breaking it free. Any spills will drain into the bag instead of the bilge.
Before you empty your bilge think about the consequences. "The Federal Water Pollution Control Act prohibits the discharge of oil or oily waste into or upon the navigable waters and contiguous zone of the United States. If such discharge causes a film or sheen upon or discoloration of, the surface of the water, or cause a sludge or emulsion beneath the surface of the water. Violators are subject to a penalty of $5000."
This fine does not include clean up, attorney and local violation fees.
The enforcement of the laws shouldn't be our deterrent, our love for the environment, life forms and the beauty of a pristine world should.