Akuei to Gacchinpo

Surreal gag-packed story about a Japanese boy named Takeshi who, after having immense toilet troubles, takes a bathroom break from class and ends up sucked down the toilet into an underwater world. There he meets a spunky girl named Jasmine and is dragged into her organization, UNCO, tasked with fighting the evil PIPS, a group bent on destroying the world.

Both Takeshi and Jasmine often use the underwater world to travel into Springtown, Taiwan, to team up with their fellow UNCO member and good friend, Akuei, in order to take down the evil menace or deal with whatever wacky hijinks stand in their way.

(Source: AniList.co)