The negative side of boat wiring is the negative terminal on the battery. Follow the negative wire up front. You will find the ground bus. In fact it may only be a single stud ground point. Every circuit needs a return so don't overlook the fact that grounds have been attached to the aluminum hull. Not a good choice but it has been done. Sometimes the fuse box itself has a ground bus. Since a large (#10 or #12) gauge wire typically runs from the positive terminal of the battery to the fuse/breaker panel, there should also be a same size ground wire routed to the ground bus on the fuse panel or to a separate ground bus stud. Look for those two wires.
