How to get the Mermaid Achievement/Trophy for Sea of Solitude

How to get the Mermaid Achievement/Trophy

To get the Mermaid achievement/trophy you must swim a very long distance. The easiest areas to do this are those that are free of the Whale as it means you do not have worry about being eaten by it. Two spots such spots you can use to unlock the achievement/trophy are:

Chapter 4 (Hiding)

Once you are aboard the boat from the end of chapter 3 you can leave it and swim around as the daylight is not restricted to it. You just need to make sure you don't get too close to Sunny.

Chapter 11 (Solitude)

You need to have finished the game for this area. As the chapter is loading head back to the cliff and jump into the water and just swim in circles around the island until the achievement/trophy unlocks.