Bundled with the 11th compiled volume of the manga. The OAD adapted the manga's chapter 17 from the fourth volume, where Miia, Cerea, Mero, and Kurusu go to the gym's pool.
Reviews
AniInterview
Sorry, this movie sucks
SpuffyWeb
Sadly Over-hyped
Pacionsbo
Absolutely Fantastic
ChanFamous
I wanted to like it more than I actually did... But much of the humor totally escaped me and I walked out only mildly impressed.