GPS spoofing in Shanghai had been discussed informally among maritime interests for months before a formal report was filed with the U.S. Coast Guard earlier this year. The non-profit C4ADS became interested and found that spoofing had been going on for some time. Also that, over time, many of the false vessel positions tended to form a circle some distance inland.
I’m surprised some fishing vessels haven’t tried doing this. Especially, in closed areas or on boundary lines. Maybe they have? Obviously it wouldn’t work if you had on scene oversight.