To me this seems like one of those things that are technically possible, but highly unlikely.
The biggest hurdle I can see for a potential spoofer is that they would need to know the intended track of the vessel. You can’t spoof the signal if you don’t know where the ship is going. And I don’t think it would be as difficult to detect as some are arguing. If nothing else, the first time you change course for traffic you’ll notice something’s up… 