Inbi Houman na Juumin-tachi ~Omorashi Shoujo, Shikamo Shojo~

Despite the title, there are no water sports in this story. Just one girl has her toilet malfunction and doesn’t manage to reach her neighbor's in time. That neighbor (the protagonist), then invites her to get in and have a shower. Turns out, he got more than he bargained for, as heroine is a horny virgin who loses control once she’s left alone in a boy’s bathroom.. Note: Initially published under the name Omorashi Shoujo Shikamo Shojo - Chiisai Ana Demo Ganbarumon (おもらし少女しかも処女 ちいさい穴でもがんばるもん)