Items: Mythril Shard X5, Mega-Ether X5, Cottage X3, Elixir X3, Mega-Potion X2, Mythril X3, Torn Page, Crystal Trident, Orichalcum, Ansem's Report 3
Watch the scene here. Your pal Donald will use his magic to turn you all into aquatic-like creatures. You get turned into a mermaid (or whatever you'd like to call it), Donald gets turned into a deformed squid, and Goofy becomes a turtoise. Anyways when you reach Atlantica you'll encounter Ariel and Flounder. You'll fight off some Heartless in your new underwater bodies. Then be taught how to control them properly. Its a little rouhg at first, but you'll get the hang of it. Anyways the nearby shell holds a save point. Hit it to open it and save. I suggest putting Ariel in your team, as she is the most experience underwater fighter you have.
Now follow the Tridents painted on the walls to make your way to Triton's castle. Head through the next area, the Undersea Valley, fighting and casting the spells that match the color of the shells to open them. Follow the Tridents down into a trench then into the next area. Make your way through this area, the Calm Depths, and again follow the Tridents. In the Undersea Cave area you can either go straight up or fight, I say just fight for the EXP. This is the Undersea Gorge, anyways head through the hole the Tridents point to and you'll be in the area leading up to Triton's Castle. Scan the bottom sides for treasure. Enter the castle to appear before Triton's throne. After a scene, drop down to the shell below and hit it to reveal a save point.
Now head back to the Undersea Gorge and clear out the enemies, ride the dolphin a bit then head to the side of the gorge with a rock blocking what looks like a cave, push it aside to reveal Ariel's Grotto. Inside open all of the chests. After doins so, go ahead and leave the Grotto. Head to the Undersea Valley and you should encounter a Dolphin in this area. Clear out the enemies then ride the dolphin until you reach the sunken ship. Raid this area for treasure then enter the large ship. Search it until you find the Crystal Trident. Leave the ship and if you want the EXP take out the Shark, it has high HP so be careful. After that look for the broken off front end of the ship to the left of the boulder with the symbol on it. You'll see something behind, but Sora won't say much. Now head to the left and enter the hole leading to the Undersea Gorge. When you get there, hit the glowing pedestal to unleash a geyser. This will shoot you up to the Cave. Now head back to Ariel's Grotto and place the Crystal Trident into the indent in the wall. After a nice little scene head to Triton's Throne.
After the scenes here and once you save head back to the Sunken Ship, enter the area near a ship that Sora gets the ? at. Sebastian will open it for you and you're now in the Den of Tides. When you reach a fork in the road take the right path to reach a save point and a Mega-Ether. Prepare at here for the upcoming boss battle, equip Thunder, Fire and Blizzard on your spell shortcuts and have plenty of ethers. Now head back to the fork and take the left path to the boss fight.
After the battle you acquire the shared ability Mermaid Kick. Now, from the sunken ship area, head into the Calm Depths. Using Mermaid Kick (equip it) head across the hall swimming against the currents to the hole on the other side. Now put Aero in your magic shortcut and head into the "???" area for another boss.
For winning the battle, Thunder is upgraded to Thundara and you get Ansem's Report 3. Sora will then seal the Keyhole in Ariel's Grotto. You then receive the Crabclaw keychain.
Head on back to Traverse Town if you need to re-stock on items. If not, go ahead and head toward the 6 star battle area.