There's a lot less water than I imagined.
Ocean Storehouse (地下室, Chikashitsu, Underground Room) is an area accessible from the pier in the City Limits.
The Ocean Storehouse starts off in a set of sewage drains, where the puddle of filthy, green water serves as an entrance and exit. Going eastward is a hall with a series of vending machines next to a spiral staircase.
Going downstairs from the aforementioned spiral staircase is a corridor with numerous posters on the wall, featuring a green creature and some numbers. From the left staircase of this floor, going downstairs leads to a small room with some vending machines and a gate that cannot be opened, while taking the right staircase leads to a cross-shaped room which is a dead end.
By going upstairs from the entrance, then going downstairs from the right side of the two succeeding corridors, and going downstairs from the left staircase on the third corridor that you encounter, you will encounter a lobby and an inaccessible door next to some windows. Taking the staircase on the right side of this area leads to the deepest section, which appears to be a pier blocked out by a traffic cone and some barricades.