The Ocean Planet! Become a Mermaid and Swim Swim - S1.E27

Arriving on Yanyan's home planet Pururun, the girls receive pearls from Yanyan allowing them to transform into mermaids and swim underwater. However, they are soon pursued by Eyeone, who swears vengeance against Yuni for deceiving her and attacks with a robot. Although Yuni is shaked up by Eyeone's words, Hikaru gives her the encouragement to beat her. The girls eventually arrive at the core of the planet, where they can get their rocket repaired.