For this achievement you have to visit every location in the game. This means that you have the ability to fast travel to every location on the map. To do this, you're required to visit every listed island, city, diving location, AND fort. You'll know that you've already visited the location when the map indicator is either white or gold (for locations) or lime green (for forts), allowing you to fast travel there.
Areas not yet visited will show on the map as either a grey map icon or a question mark. Red forts are also unvisited. You will also not be able to fast travel to these locations. Diving locations are required for you to at least dock and go into the water, but no treasure collecting is required. Other locations you just need to stand ashore and you'll get an in-game notification telling you that you've unlocked a new fast travel location.
Things such as naval convoys, whales, sharks, collectables, and legendary ships are NOT required for this achievement.
When going for this achievement you'll pick up the SEVEN DEADLY SEAS and KING OF THE CASTLE achievements.
Note: This achievement is not obtainable until you're at least halfway through Sequence 10 where Long Bay becomes available.