I agree with the advice to operate the switch and see what happens. Your boat should have several original switches and they should be labeled so the additional switches should operate something else. Or, they may not operate anything, they may have gone to something that's no longer on the boat.
As for tracing wires, that is usually a lot harder than it sounds. And while the tracing device shown above is a handy tool for an experienced person, it often doesn't work well for an inexperienced person. The signal can bleed from one wire to other wires. I used to use one in my job, I know it's limitations.