Hello Muhamed!
Forget flight plans. We've got people in classic boats and worse even classic cars taking off around the country!! Let's just toss 'em all in jail. Just to be on the safe side.
It was a mistake, the gentleman owned the boat, he probably launched it and got lost (you know 1750s submarines probably didn't have the best radar) and you're ready to label him a terrorist because he may be a 'clean' person.