Razjah
Explorer
Cannons will have a range of 50 ft, require at least 1 person trained in cannons to use, 2 people to load and fire, 1 full round action to load. The attack bonus for a cannon will be ~ +3(+6 with 2 people trained in gunning). A normal cannon ball will deal 1d8 damage. I plan to have 2 types of cannon balls, an explosive ball which will deal 3d6 fire in a 10 ft radius and 1d6 bludgeoning from the actual ball, and a blades ball, which deals 2d6 and ignores hardness of wooden objects.
I would greatly increase the cannon ranges. At least 200 ft. They may be inaccurate if you want, but at 50 ft the cannon are more than useless. It would be better in this situation to have blunderbusses and bind the ships. Then the crews can fight each other.
The exploding cannon ball is odd. I would simply drop it, there are plenty of spell effects you can use alchemically.... actually, you could have magically charged weapons. A lightning ballista or a fire cannon. Then you can just moidfy fireball/lightning bolt/cone of cold/you spell of choice/etc. to be your attack of choice with a weapon. These would let you not need to worry about the extra rules for cannon. If you really need a method to attack a ship, just use normal hardness for wood and give a decent amount of HP to a ship's hull section.
If the battles are really meant for the ships to sink each other, then have armored hulls or attacks that can ignore the hardness.
This lets the ships be able to fight off dragon turtles, sea serpents, and much of the monsters of the deep. Or people stick to the shallows and run from the monsters, making long voyages slow or extremely dangerous.
If you give a bit more about the world then some more advice can be given. Waterworld didn't have sea monsters, just people fighting. Same with the Pirates of the Caribbean films. If you want more monsters- how common? how easy to fight off? What levels of play are you aiming for? (I recommend E6) but high level play will require different sea hazards when mages can control the weather.
Hope this helps!