When I worked at a marina, we used to use muratic acid (for pools), it works good but is nasty to use. Put it in a spray bottle, spray it on let soak, presure wash it off. It ussally worked better if the boat was dry. Don't breath the fumes, wear gloves, have extra spray bottles, as it will eat the spray mechinisms up. Probably do it on driveway also, throuhly wash drive to dillute when finished, might kill the lawn, I don't know.