Don't bother upgrading:
• The Rowboat
• The Crew's Quarters
• The Jackdaw or Kenways Appearances
Always look for the lost at sea crates and grab them. It seems silly, but for some reason there are more people and crates lost at sea than you can shake a stick at. You’d be amazed at the amount of loot you can pick up just roaming the sea.
As you travel in the north seas, there will be tons of gunships and the like roaming around. To start, look for a lone ship and take it. If you're feeling tough, the lone ship you want is a Brig. it's tough, loaded, and...yea tough, but well worth it. Always look for the ship with the most metal and wood on board as you will need loads of each for upgrades, and you can always still earn money by selling the rum and sugar you pick up.
For now, board the ships you take down and get the resources from it. It won’t be much, but it takes a bit to get up to the good goods. Put all the rum, sugar, and cloth you earn towards storage space for more sugar, rum, and cloth. Cloth is only used for appearances and the crew upgrades, so sell it when you can. Keep boarding ships until you are able to upgrade the hull and the cannons some.
On The Way Up
Once you start taking down ships and earning money, start throwing your nefariously earned coins into the hull, first and foremost, and then your cannons and heavy shots. Later on you’ll want Chain-Shots and the ram addition, but first things first.
To the South Side
At this point you should be "medium sided". This should be about the time you head to the south waters. The south waters have the Man O’ War’s, Frigates, and the Galleons; the large ships that give a worth of trouble. At this point, you can consider yourself a bad ass. Now is when you don't ever want to waste time boarding a ship again. With the time you spend boarding, you could run over four other ships.
Always use your “Spyglass” to look at the ships in the area because even Galleons can range in levels by twenty points which is crazy, as can the Man O’ War’s. Always look for the lone ships as usual and by this time you shouldn't even bother boarding. It's harder to complete, more time consuming and overall just not worth the trouble. Also keep an eye out for the ships that have Reales on them as well as resources. Even though you only get half the resources when you sink and loot, you will still get the full amount of money.
Now that you are in the South warm waters, roam like a lost doggy and tear down each boat you see. You should be able to take on one Man O' War face to face or a Galleon and the three Schooners that are following it. Once you reach this point, it's up to you to ravage the seas. You should be able to basically be a terror north, west, south, and east, and the Hunters don't pose a problem at all.
Until you try to take on the Legendary Ships...