I am thinking it is anti-fowl paint. It is used to keep "things" from growing on the hull that is below the water line. It will fade in direct sun, but that stuff is hard to get off, so it might work for you. I have it on my boat and repaint it every so many years. It actually wears off sliding through the water over time. When it is first applied, it actually helps the top end speed of my 30' boat.