Honestly, unlike most OVAs I have watched, I felt like this one was better than the entire original anime series.
It covers only two days in the life of Teko, Pikari and the two middle school friends of Teko, Chizuru and Akane, who came over to visit her during the summer vacation. This short period of time, however, was enough to well develop the two new characters we are introduced to. I fell in love with both of them, mostly Chizuru.
The story also focused more on the characters feelings and the time they spend together other than diving (although it was also part of
it), which I liked a lot, since the girls are all really cute as a group. The way they share their feelings and talk things out made my heart really warm.
All in all, it was one of the best OVAs I've ever seen. It was pretty good both as a complement to the previous anime series as it was as a story of its own. Definitely recommend it.