Thanks for all the replies which collectively are very helpful. I
didn't make it clear in my original posting that the outboard does have
an electric start so I suppose that it does act as an "earth" - but
presumably only whilst it is in the water. When sailing or at the dock
the outboard is raised. My problem is getting my head around the
concept of grounding connections on a boat (as opposed to land-based
earth connections or vehicle grounds) - but, if I read your replies
correctly, it seems that it is not that important.
Thanks again, everyone.