At the moment, sinking an enemy ship with just cannonballs seems almost impossible, unless you're a galleon broadsiding a sloop. Most of my playtime is spent as a solo player on a sloop searching for and fighting other ships, and many times I've sustained damage that left me thinking "I should have sunk from that". The only times I ever sink is if i'm either killed or knocked off my ship. One time i ended up sailing off into the Red Sea, and I got to something like 7-8 holes (most of them below deck) before the water overwhelmed me (and that was only because I briefly went up to adjust the steering wheel). And to reiterate, that's me solo - with two players, I suspect nothing less than all possible holes would be needed to sink your ship, which isn't really going to happen from just cannonballs.
The solution is pretty simple; either increase the amount of water intake per hole, or increase the amount of holes you receive per cannonball (it's not unthinkable in a sort of in-universe sense, that a cannonball would pass through a ship and cause two holes in the hull, for instance).
I understand that a significant portion of the playerbase dislikes PVP, but I don't think such a balance change would have a significant effect on these players. It doesn't affect your ability to run away, after all, and in my experience, players seeking to avoid PVP either take no damage or all the damage (because they're parked at an island and a galleon just decided to say hi to their broadside).
