Since I have my pontoon in saltwater 24/7 I spray the pontoons and underside with freshwater after use. My boat has been in for 2 weeks now and today when I sprayed under the boat I saw that the water I sprayed on top of pontoon where the M brackets are came down with massive amounts of black residue. Is this stuff due to beginnings of oxidation or could this be an accumulation of dirt from sitting at dealer? I sprayed for close to 3 minutes and water never cleared.