If you don't see the boats right away in the quest area, shoot the canons around close to shore. You might have to wait for them to make their way to shore, the boats might be invisible but eventually you will hit one. If you turn on "Track Quest POI's" (click the magnifying glass on your minimap), it will highlight the area where the boats path to shore - this will help you better guess where you might be able to shoot the boats if they aren't visible to you.
Tried to do this quest today on my druid and everything went fine until I got up to 8/10 boats sunk.
I tried the other canons and I shot a couple dozen cannonballs but nothing seemed to happen so I decided to restart the game to see if that helped. I can't confirm that restarting the game helped because I did not see any boats at first after logging in, but after shooting the cannon some more I did find one of the spots to hit (where the "invisible" boats come to rest after spawning and making their way to shore).
The only approved screenshot included here at the moment only shows a side shot of a cannon and you can't see where the relevant boats are in the picture. I submitted a screenshot that I took when the boats re-spawned and came to rest after making their way to shore -- that picture should help give you an idea where the 'invisible' boats end up, so you can hit them with your cannon and hopefully get some more to spawn. Good luck!