Common sense will tell you that fighting single ships will be considerably easier to fight. And yes that is true. So first thing you should do (if you are bigger than the ship you are fighting) is to destroy their sails with chainshot to keep them from running away. Once you do that, hammer their hull with normal rounds (or double shot if you are looking for more damage) until they sink. Or you could use grapeshot to kill their crew and capture the ship for more profit by selling the ship you captured (this process will take longer normally)

Fighting fleets will be considerably harder than fighting individual ships. Normally every fleet has one or two larger ships then more smaller ships (ketchs, brigs etc.) What you need to do is take out the biggest ship first, utilize your mortar (if you have one) to deal massive damage to the hull. Dont attempt to board any ship while in combat (chance of loosing valuable crew that could slow down reload times etc.) sink each and every ship while fighting fleets. Once you destroy the bigger ship or ships, go after the small ships. At this point in battle you are practically victorious (unless your ship is crippled) so you dont need to worry about loosing.

Simply cargo vessels with military escorts. Obviously, you should attack the military vessels first. Dont try to board the ships (as I have covered before) after you finish off the escorts, the cargo vessel wont be much threat. Not much to say on this topic, mostly common sense.
For an example of attacking a convoy, see this Youtube video made by me!

The biggest challenge in combat is attacking towns. They are protected by at least 1 fort and a fleet (the bigger the town, the bigger the fleet) Go after the fleet first, stay away from the fort/forts for they have cannons and can assist the fleet in trying to kill you. As I have covered in the fleet section, follow the same course of action. You need at least 30 mortar rounds to destroy a fort, after you destroy the fleet. The forts wont be much of a threat (as they dont have much armament) so you can easily destroy them.

Multiplayer games are a mixxed bag of tactics. What you should first do is destroy their sails, no matter what size of ship. No sails means hard time turning. Once you do that, load your fore and aft cannons (front and back cannons) with grapeshot. So that way, once you're given the chance you can widdle down their crew but not sacrifice your broadside cannons the ability to do damage without needing to reload again. If you are loosing, retreat and try to use your mortar (and back cannons) to try and keep your distance. If that fails, I cant help you :P

I hope this guide helped you know what to do in various combat situations.

For The Pirate: Carribean Hunt videos, go to Irish Cabbage on Youtube
My channel might be covered in salad tutorials :P

Anyways, good luck and Godspeed!