The Sea is an underwater world accessible from the Red Forest.
Features
This deep, dark world is home to many stone pillars and bright coral scattered around the area. A cluster of stones can be found resembling the Uneven Dream logo. There are also a set of tiles on the ground in the shape of a crescent moon and a star in one area, where a floating crystal resides. Interacting with the crystal will teleport you to the Mermaid Mansion.
Mermaid Mansion
This regal area is comprised of a large number of rooms to explore furnished with bookshelves, couches, and vases. There are many bedrooms inside of the mansion as well. One bedroom in particular is noticeably different from the rest, with three large windows shining light inside. Using the UV Light here will reveal a ghostly mermaid, who will give you the Mermaid effect upon interaction.
Directions
Totsutsuki's Nexus → Dream Palace → Red Forest → Sea