In Kingdom Hearts I
Sora and his friends arrived in this underwater world in the middle of their adventure to find a peaceful world rulled by King Triton. Ariel however wants to become a human being and Ursula will trick her to the point where King Triton is now a prisoner from Ursula. Ursula will have to be defeated and you will have to do it twice, the second time being more memorable as she becomes a giant. The world is different than the others because Sora himself becomes a male 'mermaid', Goofy a turtoise and Donald a squid. Opening up a world of possibilities and this is one of the most original world in the game.
In Kingdom Hearts II
In the sequel, the makers prefered to make the world more loyal to the real story so you won't find any Heartless nor any members of the Organisation XIII. When Sora arrives, you will automatically see the difference as the game became a musical sorta game. You don't HAVE to play this world though for people that don't like it but some rewards are pretty good so hardcore fans will pass through it. Ariel is again obsessed by the surface and her father will try to make her forget it by making her play a musical. Ursula (she's still there) will trick her and make her become human. You will have to fight Ursula again in a musical fight.