Ureshon!

A boy falls in love with a girl, whom he witnesses urinating in public. Unable to confess, he is gifted with a deus ex machina when the girl suddenly transfers to his school. Never minding the coincidence, he immediately calls out to her, overjoyed to find out that she wet her pants the moment she sees him. Hijinks ensue as the two strike up a deal to partake in their darkest, wildest urine-based fetishes. While the two engage in water sports, drama ensues as they begin to fall in love with each other instead. (Source: MangaDex, edited)