Though each one is small in volume, oil spills from small vessels add up. In Washington State, when you multiply this volume by the thousands of fishing and recreational boats on the water, they compose the largest source of oil pollution in Puget Sound. How do small oil spills happen? The two most common causes are spillage during refueling and bilge discharge, when oil accumulates along with water in the bottommost compartment of a boat and then gets pumped out..