I had a friend ask me why the player would choose to go for either cannonballs or loot, because there wasn't really an advantage to either of them. They did the same thing in the end really.
So the change I have made is this:
Rather than having ship lose health when they are hit with cannonballs, they will now only lose loot (if the player has already collected loot for their ship that is). So now, the only way to win is to fill your ship with loot, and you can decrease the amount of loot your opponent has by shooting at them with cannonballs.
This also means that the player must choose between interesting their own loot stash, and damaging their opponents' which could lead to some interesting matches.