Mermaid Kick

Mermaid Kick (マーメイドキック) is an ability that appears in Kingdom Hearts and Kingdom Hearts Final Mix. It is a Shared Ability that allows Sora to swim faster and more powerfully by repeatedly pressing ( in the original Japanese version). It is required to swim against the current in the Calm Depths, but it can be used anywhere in Atlantica, and even in bodies of water in other worlds.

Kingdom Hearts and Kingdom Hearts Final Mix

 * Sora learns Mermaid Kick after defeating Ursula in Ursula's Lair.

Kingdom Hearts II and Kingdom Hearts II Final Mix
In Kingdom Hearts II and Kingdom Hearts II Final Mix, the gameplay mechanics of Atlantica are simplified. In lieu of Mermaid Kick, Sora has access to Dolphin Kick (ドルフィンキック), which is an action consisting of a single forward kick followed by a backflip, executed with ( in the Japanese version).