This trophy/achievement is only in the original version of the game and not in the Definitive Edition. At some point in the game, you will acquire a ship, although you are restricted to the inner sea at that point. Not long after that, you will gain access to the outer seas, which means you will be able to explore a lot of the world. Throughout said world are small islands that you can make port at, with there being seven in total. At the point where you have access to the outer seas, you can land on six of the seven islands, with the last one requiring the Lorelei’s Harp key item. The islands are as follows:

• Insula Australis
• Insula Incognita
• Insula Borealis
• Insula Occidentalis
• Insula Orientalis
• Insula Algarum
• Insula Centralis (requires Lorelei’s Harp)