A lot of people struggle to learn how to aim cannons, i will start this by saying no tip is better than practice, but they can give a baseline where to start!
When you choose cannons, its key to choose 1 type and stick to it on your ship, changing cannons will mess with your aim every time you switch.
My recommendation however is default because the rings on it are a good guide for distance! If a ship is fairly close, have the front most ring in line with their ship, if they are far, the back most ring should line up, and middle ring for the middle ring. But as long as your cannons arnt too bulky and dont cover your view, you can learn aim well enough!
Dont fire as soon as the enemy ship is in front of the cannon unless you are moving at the same speed as the enemy. Since cannon balls keep your ships forward momentum if you are moving faster than a ship that is ahead of you, you can fire just before you are lined up and the shot can still hit!
On the note of cannon balls maintaining momentum, turning shots tend to be the toughest for most players because your cannon ball maintains your forward momentum if you were moving strait forward, so it makes the shot seem to curve heavily forward or back. What you need to know to land these shots is if you are on an inward turn around an enemy, your shots will seem to curve ahead of your ship, so aim slightly behind the target, and if you are turning away from the target, the shot will curve behind so you will want to aim slightly ahead. Practice is very needed to get down how much you need to adjust based on how sharp you are turning.
As for small tips:
*Dont just aim for the hull, try to hit multiple spots, and if you see some one on the enemy deck or cannons, it can help to aim for them.
*If you are in a position where you cant land a shot, grab ammo, there is nothing worse than getting on cannons when you have a shot lined up only to have 2 shots.
*Dont spam every special ammo you have if you arnt sure of the aim, it seems obvious but many people throw every curse, chain and blunder they have only for them all to miss!
If any fellow naval enthusiasts have any other tips, feel free to share!