Mermaid Kick
The Mermaid Kick is a Shared Ability in Kingdom Hearts, and it "reappears" in Kingdom Hearts II under the name Dolphin Kick. Sora can only perform the move while he is in his Merman Form, which limits its use to the watery world, Atlantica.
Kingdom Hearts
In the first Kingdom Hearts, the move is learned after the first fight against Ursula, and makes Sora, Goofy, and Donald Duck able to swim at incredible speeds by holding down 
(or 
in the original Japanese version and Final Mix). It is an ability that must be required to be able to swim against an exceptionally strong current in order to reach Ursula and fight her a second time, thus proceeding in the storyline.
Kingdom Hearts II
In Kingdom Hearts II, however, the move is "remembered" due to events from the first game, and thus, does not need to be re-learned. However, the original properties have been lost due to the change in gameplay in the Atlantica world. Rather than continuously swimming fast, Sora does a single thrust forward followed by an upwards loop. It has no real use and is purely aesthetic.
Trivia
Mermaid Kick is almost like a counterpent of Dash, another speed ability that Sora can do in his Lion Form.
