Islands can either be small or large in size, the former only taking up a single node on the map, whilst the latter take up four nodes.
There are currently 7 types of islands discoverable, all unique in their own way.
The most common type of Island is the Uninhabited Island. It has no Pirates, more often than not houses an NPC or two and is a good source of common materials. The player begins Salt on an Uninhabited Island.
Uninhabited Islands can be Small or Large in size.
Desert Islands are a relatively rare occurance, and are the smallest island type. They will always have a Treasure Hunter, along with a Desert Island Treasure Cache buried beneath sand and marked by an X.
Desert Islands are the only island type that cannot be Large in size.
The Pirate Island is the most common of the three types of islands that contain Pirates. Depending on their size, they may have 5 to 15 Pirates. Occasionally, a Pirate Village may appear on them, with two crates of common loot and sometimes a Bronze Chest. Pirate Islands always have a Bronze Pirate Challenge Chest.
Pirate Islands can be Small or Large in size.
The Battlemaster Settlement is the second tier of pirate inhabited islands. They generally have 15+ Pirates on them, and are unique in that their grass is orange and tree stumps are scattered about them. They will always have a Silver Pirate Challenge Chest, as well as Pirate Villages.
Several types of pirate spawn here, including the Pirate Battlemaster and Pirate Commander. Until they've been defeated, each Battlemaster Settlement will have one of the four Boss Captains: Captain Bobo, Captain Bulgeye, Captain Buffbeard or Captain Rosebernard, who can be found in any of the Pirate Villages on the island.
Battlemaster Settlements can only be Large in size.
The Pirate Stronghold is the last tier of pirate inhabited islands, only appearing far from the map's origin. More than 50 Pirates populate this type of island at a time, and are the only island where the player can accrue a bounty. They are the largest type of island, and contain a pirate fort in the center. They will always have a Gold Pirate Challenge Chest, though it can be difficult to unlock as these islands are extremely populated.
Pirate Strongholds can only be Large in size.
The Jungle Island is a rare type of island, appearing just as far from the map's origin as Pirate Strongholds. They can be extremely dangerous, as environmental hazards are scattered around them, as well as some of the toughest enemies in the game, like the Poachers or Tigers.