This is another world that is very straightforward. It will be available to travel to after you finish either Halloween Town or Atlantica. Any time you see a shadow Sora, be sure to kill them - they have relatively low HP and give a ton of XP / good item drops.
When you start, walk through the only door you can & fight everything. Climb up after killing everything, and go through the door across from the yellow trinity. Drop down underneath, fight everything, then climb up the other ladder. After the cutscene, climb up the shelves and go through the hole in the roof. In the new room, use the green trinity then save and prepare for a boss fight.
Anti-Sora can be difficult if you're not sure what to do. If you're having a ton of trouble, use Aero to reduce damage taken and be sure to have ethers for replenishing mp. Don't get greedy with your attacks - do 1 combo, then dodge roll away. If he falls into the floor, he'll appear behind you and attack - just dodge roll away here as well. When he splits in 3, if you hit the wrong one - dodge roll asap as the right one will come in to hit you.
After, open the treasure chest, then the hatch (you should see the outline for it on the floor) and drop down for another cutscene. After, head back up and out onto the deck for a cutscene and some fighting.
First, you'll have to beat a few enemies for another cutscene then a boss fight. Take your time with these enemies so you enter the boss fight with high HP & MP.
Captain Hook boss fight is fairly straightforward as well. He is really easy if you use some strategy against him, and you can pretty much ignore the ship that is flying around. Stay near full health, and hang back from him a little. As soon as he finishes a special attack, dodge roll to him, do a combo, then go away. Rinse & repeat until he is dead. If you're struggling, Aero & ethers are your friend.
After the fight, you'll be at the clock tower. Run around the platform you're on for a treasure chest, then fly down to the clock faces. 3 of the 4 should be on the same time (it was midnight for me), and the other will be off. Move the 1 different clock to show the same time to reveal the keyhole and a few cutscenes. Board the gummy ship for the achievement!
The text guide above was written by me. If you're still unsure, feel free to check out the video below that was made by someone else. I'm bad with videos, so I'm going to youtube for this one. Good luck, and if you have any questions, feel free to ask in the comments!