Kingdom Hearts/Atlantica

Atlantica is a world based on the 1989 Disney film The Little Mermaid. A cutscene will show that things will be a little different here. The first thing to do will be get used to the new controls.

Swimming
Use psx: Neutral lstick to swim around, as in walking. Press psx: C to rise and psx: S to dive. Release them to stop rising or diving. After you've got the controls, your task will be to follow Flounder around. You should know when you're finished after a short cutscene where Sebastian tells you you're doing a good job. Next will be self-defense, where the tutorial will be interrupted by a Heartless attack. These Heartless will look similar to jelly fish to match the theme. Attacking is exactly the same as attacking above-ground, other than adding on diving, rising, and moving around in the new aquatic atmosphere. After the battle is over, hit the clam with your keyblade to find Sebastian, Flounder, and a Save Point.

To the Palace
After opening the clam, you'll be asked to go to the Palace with the group. Trident markers are lined along the walls, pointing out the way to go and making the trenches easier to navigate. You will now have the option to choose Ariel as a new party member. Follow the trenches to the Undersea Valley. Follow the markers starting from the right of the circle, then go to the Calm Depths. The strong current will make it impossible to swim against it, so you'll have to follow the current into a pathway on the right. Keep swimming to the right to make it in during the strong current. After making it in, go to the other side for another path. This leads to the Undersea Cave. Then rise up to the Undersea Gorge. Follow the markers, the last one leading to Triton's Palace. Here, just go straight forward to Triton's Throne, where you'll see a cutscene.

Fighting the Current
After the cutscene, backtrack to the Undersea Gorge. Here, a secret path behind a boulder will be pointed out through a short cutscene. Go to the boulder for a cutscene in Ariel's Grotto. After leaving the Grotto, go to the Undersea Valley. Dolphins are constantly swimming around the area. Try to hitch a ride on one of them, then, while still on the dolphin, get to the Calm Depths. The dolphin will be able to swim against the current. Go to the last opening on the left. This path should lead to the Sunken Ship. In the ship, after getting attacked by the shark, open up the treasure chest for a Crystal Trident.