Federal Law: Prohibits the discharge of oil or other petroleum products into the waters of the United States.  Boats 26' and longer are required to post an oil placard near the engine:

These placards are available from marine supply stores (including BoatU.S.) for about $1. 

By law, any oil or fuel spill that leaves a sheen on the water must be reported to the U.S. Coast Guard's National Response Center at (800) 424-8802.

It is against the law to use a detergent or other chemical on a petroleum spill to disperse the oil or sheen.  These products can cause the petroleum to sink in the water, causing more harm to marine animals and bottom sediments.