Flipper

Flippers are accessories that allow swimming in water. They also allow swiming in lava, but is not recommended, as lava does 100 damage/sec. Unless you are using an Obsidian Skin Potion, which gives you lava immunity, it is not advised to try swimming in lava. Swimming is achieved by repeatedly hitting the jump key. They are found in chests, with an increased chance of being found in underwater chests....

Their usefulness is debatable, as Grappling hooks, Rocket Boots and Clouds in Bottles can replace its function. However, it is a big help to newer players who do not have access to these items.

Upon starting a new world, there is a chance that flippers will spawn in a chest in one of the oceans near the edge. The player can start a small world and go east or west until they reach the sea. At the bottom there may be a chest with the flippers (or a Breathing Reed).

If you reach the end of the map and the chest is not there, then it may be on the other side. Otherwise, the best bet to find this item is chests in underground lakes.