so my boat sat for probably 4 or 5 years and the last 2 with about a foot and half of water inside. It looked like a pool that had sat that way for a while inside. complete with scum line after I pumped it out. Here's the problem. I've cleaned it with bleach, clr, elbow grease and a SOS pad, and nothing I try gets the nasty off. it seems to have gone down into the fiberglass. Only thing I can think of is to pump water back into it to the line and add about a gallon of bleach and let it sit to see if that gets it.