How do I avoid "discharging oil"?

[jsc7]

A naive reading of the boating laws suggests that it is "illegal" to discharge oil into US waterways, but simple logic dictates that every single boat over about 25 feet in length discharges oil.

For example, residual oil and other chemicals used on a boat will collect in the bilge, then as soon as somebody takes a shower, the bilge will fill with soapy water and discharge. Therefore, the boat will be pouring soapy water mixed with oil and other chemicals into the water.

What is the story on this? Everybody just knows this and ignores the law and it goes unenforced unless the oil discharge is large?