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.
Use to swim around, as in walking. Press to rise and 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
The oddly familiar Crystal Trident
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. A dolphin is constantly swimming around the area. Try to hitch a ride on it, 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 Glut the Shark, open up the treasure chest for a Crystal Trident.
After leaving the interior of the ship, the shark will attack you - it will now attack at random times until you reach a certain point in Atlantica's story.
Glut the Shark
Glut's usual attacks will be lunging at you with its teeth. These attacks should be easily avoidable. After a lunge, Glut will stop in place to prepare for another, giving you time to attack and then dodge before-hand. While Glut is still trying to bite you, going near it will get you hit. Try to use magic in these instances. Glut is weakest against Thunder.
You can now leave the Sunken Ship to go back to Ariel's Grotto.
 The Trident
After a long cutscene, go back to Trident's Throne for another cutscene, where Ursula gets herself involved in the world's story as the main villain. After the cutscene, get back to the Sunken Ship by once again catching a dolphin ride in the Undersea Valley. After another shark attack, get to the remains of a ship next to a large rock with a strange design on it. Get Sebastian to help you, and the rock will lower into the ground and open up a new path. This path will lead to the Den of Tides. Follow the path on the left to the Tidal Abyss. This leads to a pretty straight-forward path to Ursula's Lair.
Ursula has stolen King Triton's trident, but does not use it in this battle. Rather, she fights by swinging her hands and tentacles, and every now and then, she will spin vigorously round the room, injuring Sora and company if they come near. This is the most difficult attack to avoid, so ignore everything else when she does so, and concentrate on swimming up and down to avoid her when this happens. Flotsam and Jetsam are also in the lair to deal damage to Sora, however, they are simple to avoid, and Ursula will use cure on them if they are both defeated, so it's easier to ignore them, and concentrate on Ursula.
It should be noted that in Ursula's lair, there is a huge cauldron in the center, filled with color-changing liquid. The color of the cauldron does not indicate what kind of spell needs to be used on it, only what kind of spell Ursula will spawn from it. Launching spells at the cauldron will eventually cause a massive amount of energy to be released, stunning Ursula and her lackeys. Ursula will also throw some kind of magic potions to activate the cauldron, however, if there is little to no inervention, Ursula will use spells of her own to damage Sora and co. The easiest spell to cast onto the cauldron is Fire, as it consumes little MP and it is the first on the Magic menu. You may lock on to the cauldron to cast the spell, though it should be noted that physical attacks will not work on the cauldron. Physical attacks will refill your MP, so if you find you're low, make sure to attack Flotsam and Jetsam, as they refill your MP more than other enemies do.
Once Ursula is stunned, take this opportunity to use physical attacks on her. It is also possible to attack her while she is conscious, however her HP will go down very slow, and she will use her spin attack after two or three hits, so it would be better if you attack the cauldron.
Her attacks are few but rather unpredictable, so the key to winning this battle is the cauldron at the center of the lair to incapacitate her. Like all other boss battles, do heal when necessary.
After the battle, you'll get a new ability - Mermaid Kick. This lets you swim against currents and also swim faster. Now go back to the Calm Depths. All the way at the end of the tunnel is an area marked with "???". Go inside to find Ursula.
In this area, Sora and company will confront a gargantuan Ursula, this time, wielding King Triton's trident. It should be noted that if Sora uses any form of Aero magic here, this will strengthen Ursula. She deals non-elemental damage which looks somewhat like Thunder; these attacks increase in power and quantity when she raises the trident. She occasionally blows air currents into Sora's chest, dealing substantial damage to him. When she has only one HP bar left, Ursula can open her mouth to use a Thunder-like laser, which will damage Sora if he gets in the way.
Her only weak point is the head, so it is best to use physical attacks on it. Try and get behind her head, and start bashing away at her. This can be done best by swimming over her head, and then come down. Occasionally Ursula will push Sora away from her head, so get as close to her as you can. Don't be shy about using the Keyblade - and do heal when necessary. Aim for the back of her head and attack from behind as much as possible. However, since Ursula is always turning to face you, this makes it difficult to hit her. When she starts to gasp as if hurt or tired, swim behind her head and chain combo attacks to the side or back of her head, and swim away when it starts to get dangerous. Magical attacks can damage her as well, but normal combos work best, since she has a triple HP bar.
As Flotsam and Jetsam have already been eliminated after the first battle, you do not risk being damaged by either of them in the second battle.
If you are in front of Ursula when she starts to inhale water, use Mermaid Kick to swim away as fast as possible until she cannot inhale anymore. If you do not, she will bite you; not fun. She always declares her other attacks - a bubble attack (avoidable with Mermaid Kick) and raising the trident indicates that she is going to form a circle of damaging magic around her. Swim away to avoid that one. Your prize for winning is the Thundara upgrade and another Ansem Report.
After the battle, you'll get the Trident back. King Triton will then allow you to lock the world with your Keyblade. After this, you can leave to the next world.