It came out! Umibozu's little sister - S1.E16

Rascal-sensei and his pupils spend the day at the beach. Some local people tell Rascal that there are dreadful jellyfishes in the sea, and Rascal goes to rescue Riiya and Shiine, who were attacked by some jellyfishes. Chacha's friends have been badly hurt and need a special flower to recover. Chacha, Rascal, Marin, and Orin go to find some, but the only shore where the flowers can be found is guarded by Umibouzu's sister Umibouko.

Recognizing Marin, the girl who dumped her big brother [episode 3], Umibouko attacks them, blocking the way to the cavern where the flowers grow. Umibouko is too strong for the girls, but Umibouzu shows up and asks his sister to let Chacha and Marin go. Meanwhile, the jellyfishes unite to form a giant monster.