The pool episode you've been waiting for actually lands the comedy instead of just coasting on fanservice. Miia, Cerea, and Mero in one confined space means character friction you don't get in the main series, where everyone's too busy orbiting the protagonist. It's lighter than the TV show and thinner on plot, but if you're already invested in these dynamics, this is exactly what you wanted, the girls actually reacting to each other instead of just existing in the same house.
Kimihito lives with four monster girls under Japan's new cultural exchange program, a domestic arrangement that swings between domestic bliss and absolute chaos. When the household decides to visit the local swimming pool, what should be a simple outing becomes an obstacle course of mishaps, misunderstandings, and the kind of trouble only a lamia, centaur, mermaid, and everyday human can create together. The comedy mines genuine awkwardness from the girls' monstrous traits colliding with normal public spaces. Miia's serpentine body causes pool logistics nightmares. Cerea's equine lower half presents obvious complications. Mero's tail makes her the only one actually built for water. Meanwhile, Kimihito finds himself managing everyone's expectations while trying not to cause a public incident. The OVA captures the show's bread and butter: romantic tension undercut by slapstick, fan service wrapped around character dynamics that have actual warmth, and the simple pleasure of watching these peculiar household members fumble through ordinary situations. It's lighthearted chaos with genuine affection underneath, proving that monster girls and public pools were never meant to coexist peacefully.