The floors in the Shipwreck are called Layers. There are 18 Layers in the dungeon. The Gate key item is called the Hook, the locked door key is called the Shipcabin Key, and the Back floor key is called the Flapping Fish. Layer 18 is a regular dungeon floor that requires the Music Box Key to access the Boss room. After entering the boss area, Layer 18 becomes the "Church of Memory."
The Shipwreck has a dark history. La Saia, the Queen of Queens, declared it her favorite ship. On her wedding day, her groom did not show up. She decided to end her life by drowning herself, as a result. The townspeople sank the ship in honor of her memory, and it's been used as a storage for merchants ever since.