Atlantica
First things first, you have to learn how to swim. Sebastian will teach you and you can just chase Flounder around the grotto until you get your barrings. Follow the Trident markings on the walls and head for the Undersea Valley.
The Undersea Valley is quite large and holds lots of treasures and enemies to fight. If you’re chasing objectives just keep following the Tridents on the walls. You will eventually find yourself swiming down into the Calm Depths, which has a raging gush of water flowing through it. When you jump in the rage it will push you, so hold hard to the right side of the wall and you will eventually be in a room with enemies an large vertical gush of water. If you ended up back in the Undersea Valley you did it wrong. Continue following the Tridents into the Undersea Cave and up into the Undersea Gorge. The Gorge is similar to the Valley where it is large and fully of treasures and enemies. Continue following the Tridents to Triton’s Palace and then further to Triton’s Throne.
When you get back to the Gorge, Ariel will show you the entrance to her Grotto so go there. Just run into the rock and Sora will open it. There are several treasure boxes in the Grotto, one of which is a Torn Page so i recommend grabbing that. When you’re ready to leave talk to Flounder and he will tell you about a big fish who swims against currents. That’s the Dolphin that now swims around the Gorge and the Valley so go ride the Dolphin. The Dolphin in the Gorge will only swim you around the gorge so you actually want to go to the Valley to ride the dolphin.
the Dolphin will swim you to the Sunken Ship which you need to investigate so head down to it and swim down into the stair case. The shark will smash through the window but make sure you go get that treasure because it’s the Crystal Trident. Swim back outside. The shark will be around and you can fight him. He’s not that hard and is worth good experience. He doens’t die though he will just swim away so be mindful of his presence here always.
Swim back to Ariel’s Grotto and insert the Crystal Trident into it’s holder. After the abuse that Triton gives his daugher is over head back toward Triton’s palace to find Ariel. Head back for the Sunken Ship. Now that you have Sebastian with you you can swim down to where the broken front of the ship is smashed against the rock face. Behind it is a switch and Sebastian can go push it for you revealing the path to the Den of Tides.
The Den of Tides has a fork in the road. If you go right you will reach the Cavern Nook that has a Save Point,You must go right before continuing as left will take you to Ursula.
BOSS: Ursula
hp: 400 exp: 1300
So this fight is a little weird but not really that hard. Ursula will float around doing her little spin attack and cackling like a freak and mixing up spells in her cauldron. Flotsam and Jetsam her eels, will float around doing some damage as well. Basically you want to cast Fira or Blizzara at the cauldron to cause it to backfire on her. It will backfire when she puts different potions inside. When it’s blue use Fira to counteract it when it’s Red useBlizarra. This is what weakens her down because she has massive Defense and will barely take damage when you strike her regularly. If you run out of MP use Ethers and take some wacks at Flotsam and Jetsam.
Now you have Mermaid Kick and you can swim against the current in the Calm Depths. Across from the Sunken Ship entrance there is another spot that will Lead you to ???. This is where you will finish the fight for Atlantica.
BOSS: Giant Ursula
hp: 900 exp: 1500
This is one of the tougher fights in the game for most players. Ursula is large and obnoxious. Aero helps immensely and should be used constantly. Make sure Donald and Ariel are in your party and that Donald is set to use mostly Defensive type magic. Utilize your new Mermaid Kick ablity to evade her attacks. She will always taunt you before doing one of the really bad ones so pay attention to that so you can start your escape. This fight can take perseverence so keep at it and you will get her.
Triton will tell you to head to Ariel’s Grotto to seal the keyhole so head that way and you can now make your way to Halloween Town.