Might be wrong, but would be surprised if this worked, given Chinese Navy for example frequently escorts fishing boats, so I assume if sinking ships became common China would just sink any unflagged ships as pirates ghost ships.

Sinking a ship at sea sounds like a decent way to end up killing people by accident, I think targeting them at harbor would be a better play. Sugar in the fuel, that sort of thing.

But it wouldn't so much be about the practicality of the attack, it's just about messaging. All humans are put at risk by irresponsible fishing practices, so people that fish irresponsibly should have to worry about interference from any human--not just whatever agency would pull you over and give you a ticket.