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Oldsalt has it correct... I would add simply that the ground connection to the water can be subtle... for example, it might be thought that the DC is not grounded to the water...but if the mast is grounded to the keel, then lighting, instruments or antenna might provide the DC ground to the mast and then to the keel.
The outboard is another possible location and inboards are a sure thing as they almost always have DC ground attached to engine block. I've checked my Honda, and DC ground is isoloated from motor frame.
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.