Swim This Way
To reach Atlantica you must go through the Splash Island course. You get to follow Flounder like in the first game, learning to swim once more. While you are here, pick up Two Sides #1 behind a pillar. Sebastian will lead you on a tutorial about timing your rhythm to the music, a minigame. Press X when the bar is at Yellow for an Excellent, Orange for Good, and anything else for Bad. Score an Excellent and your score goes up 1, Good is no change, and Bad takes your score down by 1. You receive the Undersea Kingdom Map and can now start the first musical, Swim This Way.
Once you clear the musical Ariel seems a bit downtrodden. Flounder finds a statue but you can’t move it until you have the Magnet spell. Head on over to Port Royal through the Floating Island course.
Part of Your World
Return once you have obtained the Magnet spell. Head into Triton’s Throne and you’ll spot two puzzle pieces, Edge #11 and Edge #12. Talk to Flounder and agree to move the statue to start the next song. In this song you will have to hit Triangle and X. Score five or more Excellents in a row to pass the challenge so be on point! You can’t move on to the next song yet. You’ll have to come back when you have a Drive Gauge of 5 or higher.
Under the Sea
After you have your Drive Gauge to 5, return to Sebastian. The key to this song is to hit Square to score at least Good, but obviously Excellent is better. When Ariel is present you can score points but when Sebastian is present don’t hit Square! The meter on the right will push Ariel up; keep her at the highest point to clear the song. You’ll need Magnera to play the next song.
With Magnera you can start the next song by talking to Ariel then Sebastian. The battle with Ursula is rather simple. You will get the normal timed "X" cues but also ones where you can just smash "X" to deal a lot of damage quickly. In the end, you defeat the Sea Witch and receive the Mysterious Abyss Keyblade .
A New Day is Dawning
The next challenge is to mix all of the previous gauges into one song. It isn’t difficult, just make sure to combo as many as possible to score the most points. For clearing this song you receive a Blizzard Element and an Orichalcum+.