Andrei - It appears to me to be a petrolium based product that was on the surface of the water. Possibly an Oil company crude oil treatment chemical , if there is any oil field production in the area of the lake. The stain appears to have attacked the surface of the jellcoat; as if it was on the surface of the water and splashed up on the side of the boat as it was under way etc. Possibly it could be neutralized or cleaned with something like Simple Green or other strong cleaning solutions that would not directly attack the jellcoat finish, Check with a local Fiberglass specialists in your area first. In a message dated 6/7/2008 10:59:04 P.M. Central Daylight Time, andreic@math.wisc.edu writes: http://www.math.wisc.edu/~andreic/P6070387.JPG **************Get trade secrets for amazing burgers. Watch "Cooking with Tyler Florence" on AOL Food. (http://food.aol.com/tyler-florence?video=4?&NCID=aolfod00030000000002)