Currently the variety of cannon balls are pretty limited in my opinion, so here is my list of more cannon ball ideas that I came up with.
Smoke shells
These cannon balls do little to no damage, but when they hit a ship or the ocean, they would create a large smoke cloud that would block a player’s vison and stops any NPCs from pursuing you.
Sunken cannon balls
A rare type of cannon ball when fired, it will fly a short distance before dropping to the sea, then it would act like a torpedo and home towards the nearest ship dealing high damage. (If you are a cannon fist user and you use this on land, it only deal high deal high damage and inflict the soak effect)
Incendiary cannon balls
When hit by it, the target would catch on fire and deal small amounts of damage over time.
Net shells
Again these cannon balls deal little to no damage, but when a ship gets hit by it, it would cause the ship to slow down as if it was on low health for 10 seconds. If it hit a player, it would immobilize a player completely for 10 seconds. (Though the player can use still use spells.)
These cannon balls are pretty gimmicky, but I think it would spice up ship combat.