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Check the manual on your outboard, some do not regulate the DC vharging voltage as it is meant only for charging, not power.
If the manual (or call Honda) does not warn that it is unregulated, the charge regulator is defective and needs to be replaced. Charging at 16+V as noted will boil out your battery and damage everything else including running lights.
If there is no regulator option for the engine, there are always ways to add an external regulator.