You may have to get a pro to look and see if it is factory or not, but some clues would be the AC panel. Does it match the DC panel? Also you could look and see if AC power was an option on your year/model. See if the wires look original to the boat are they run like the DC wires do they match the brand type wire? All these clues should answer your basic question. Get a 3 light outlet tester for the hardware store they are cheap
, use this to test the outlets that will let you know if the wiring
is basically right and safe. As for the Adapter Not sure about a cheap
one from the hardware store as I do not know exactly what you are looking at. Yes the marine ones are expense but you would know it is right and safe, personally I could not buy the parts
to make an adapter for 5 bucks so I am suspicious of what you are looking at for that price
. You could pick one up on ebay for a few bucks but like the other poster said make sure it is the molded type and not some home made thing if you do not fully understand how to check it. AC power can kill you so it is always better to be safe first. And as a final note your AC panel should have at the very least a working power indicator light to let you know when the cord is plugged in and live and it should have polarity indicator lights. If it does not have these it is likely not factory and should be upgraded.