Bonami should do it
Bill,It is good stuff, an abrasive but should not cut through the gelcoat. If you have owned the boat for a good part of it's life and know that the gelcoat has not been worn heavily, try this.That is if bonami does not work.Use 400 sand paper to take out most of the stain, Then buff it out with rubbing compound. Wax it and make this area a weekly cleaning area so that it does stain up again.The other option if it is water line grung, tape it off, ( you may consider raising the water strip some)sand it and paint it with antifouling paint( a barrier coat would be good to apply first.)Hard to say without seeing it.r.w.landau