post a picture of what you found. If it looks like a typical duplex outlet like in your house, it may mean you have the capability to connect to shore power but using a different receptacle either under the gunnels somewhere, or on the outside of the helm. Look for a hinged flap on the outside Shore power receptacles have different pin formats than a regular 110v outlet. The connector is circular as are the lugs.
Do you have a generator?
Do you know what an inverter might look like?
Do you know what a battery charger looks like?