flag ships sinking.

[Benodach]

Shooting just the guy at the helm doesn't do it. When the ship goes into "ready to be boarded" mode then all of the crew members come out on deck. If they are all wiped out with cannon fire the ship will sink. If you and your crew haven't boarded yet you don't get credit (you may still get cannon rep but no treasure).

Of course we can't rule out that people may just be seeing plain old bugs in the game but I have noticed that once all of the crew comes out on the deck anyone can sink the ship with cannons from a distance if they use them to pick off the crew.