Under the Sea is the song sung by Sebastian, Sora and the critters of the sea, to help cheer up Ariel in The Little Mermaid and Kingdom Hearts II, in Atlantica.
In the game you have to raise the mermaid icon to the top of the bar. The trick to this segment is that you have to press 
and only press it when it shows Ariel, should it show Sebastian, just let it run and go. If you succeed Ariel is closer to no longer being glum, and the story line continues.
Trigger
This song becomes available after Sora obtains a Drive Gauge level of 5 and talks to Sebastian.
Singers
Lyrics
Sora:
The sea weed is always greener
In somebody else's lake.
Sebastian:
You dream about going up there
But that is a big mistake.
Just look at the world around you
Right here on the ocean floor.
Such wonderful things surround you
What more is you looking for.
Sebastian:
Under the sea
Under the sea
Sora:
Darling, it's better
Down where it's wetter, take it from me.
Sebastian:
Up on the shore they work all day
Out in the sun they slave away.
While we devoting
Full time in floating
Under the sea.
Sebastian:
Under the sea.
Snails:
Under the sea
Sebastian:
Under the sea
Snails:
Under the sea.
Sebastian:
Since life is sweet here
We've got the beat here, naturally.
Sora:
Even the sturgeon and the ray,
Ariel:
They get the urge and start to play.
Sebastian:
We've got the spirit
You've got to hear it
Under the sea.
All:
Under the sea.
Snails:
Under the sea.
All:
Under the sea.
Snails:
Under the sea.
Ariel:
When the sardine
Begin the beguine
It's music to me.
Sebastian:
What do they got a lot of sand?
We got a hot crustacean band.
Ariel:
Each little clam here
Knows how to jam here
Sebastian:
Under the sea.
Sora & Ariel:
Each little slug here
Cutting a rug here
Under the sea.
Sebastian:
Each little snail here
Know how to wail here
Sora:
That's why it's hotter....
Ariel:
Under the water...
Sebastian:
Yeah we in luck here
Down in the muck here
Sora, Ariel & Sebastian:
Under the sea!