Under the Sea
Atlantica presents its own unique set of challenges, particularly with controlling your characters. Underwater combat and maneuvering can be a difficult thing to get a hang of. Fortunately, upon entering Atlantica, you are given a brief tutorial on how to handle the new changes to your characters. Shortly after this introduction, the Heartless show up and ruin the fun, so get rid of them and then attack the large white shell where Sebastian and Flounder are hiding in order to let them out and reveal a Save Point.
Ariel suggests heading back to the palace, which you can find by simply following the large gold trident markers engraved on the walls throughout all of Atlantica. Follow the first one to reach the Undersea Valley. This is a massive, wide-open room with lots of nooks and crannies to explore. Though there’s a lot of places to explore, for the sake of getting to the palace, follow the trident markers to an opening in the wall that leads to the Calm Depths.
The Calm Depths are actually not all that calm. There are high-speed currents preventing you from exploring the two areas to your left (at least until later), so follow the current to the right and hug the right-side wall, which will dump you in a circular cavern with an upward-facing current in the center. If you get caught in the current, it will blast you back up into the Undersea Valley, so avoid it and pass through to the other side to enter the Undersea Cave.