OVA episode bundled with fifth volume of manga.

Average high school student Kyou Sogoru is an avid swimmer living in beautiful Maihama City. He spends his days hanging out with friends, swimming, and playing video games. However, his normal life turns upside down when a beautiful and mysterious girl named Shizuno Misaki approaches him with a strange request—jump into their school's pool together. This fateful leap transports Kyou into a war-torn world where humans pilot impressive humanoid robots known as Zegas to fight against malicious aliens known as Gards-Orm. To spearhead this endeavor, humans have formed Cerebrum, a rebel organization working to prevent the Gards-Orm from eradicating humankind. As Kyou participates in combat operations, meets his fellow Zega pilots, and witnesses countless deaths, he begins to question the true nature of this world as well as his own life. [Written by MAL Rewrite]

The Straw Hats encounter a city in the middle of a whirlpool, called the Ocean's Navel, that is being destroyed by giant monsters. These monsters supposedly protect a treasure capable of granting wishes, but in the process of defeating these guardians, the crew ends up releasing another evil. (aired after Episode 53) (Source: Wikipedia)

Yamato Akitsuki travels to Tokyo alone to study in one of the high schools located within the area. He lives in with his aunt who operates a public bath solely for the ladies in the local district and begins his normal high-school life. One day, he chances upon a girl in school and is immediately mesmerized by her beauty. He is shocked when he realizes later that the girl, Suzuka, is actually living next door to him. From then on, Yamato's ordinary life begins to change little by little.