Walkthrough:Kingdom Hearts II/Atlantica

First Visit (Swim This Way)
Like in the first game, Sora and his friends transform in this world. Sora regains his blue dolphin tail, Donald re-obtains his octopus legs and Goofy becomes a turtle again. Ariel finds them, though Sora has forgotten how to swim (due to his slumber). The help text will teach you how to swim. Talk to Flounder.

A musical is near, and Ariel is acting stange. Sebastian asks you to join in on the musical, which Sora and Donald agree with. You'll be put on a tutorial round, where you must hit the X button to get a Good or Excellent. You need to have at least one to pass. You can try again if you fail.

After you complete this, you'll be ready to perform Swim This Way. You need to have at least one note to clear this song, and you can get up to five notes. Getting an Excellent grants you a note, while getting a Bad takes away one note. After completing the song, you will be re-introduced to King Triton, who asks Sora to try to keep Ariel's mind off of the human world. Later on, Flounder finds a statue from a newly-sunken wreck. However, Sora and his friends cannot lift the rock out of the way. You'll then return to the Gummi Ship, where the help text tells you that, in order to go further in Atlantica, you'll need to have the Magnet Spell, which can be obtained as early as after defeating Oogie Boogie in Halloween Town.