Gokusotsu Kraken

A young man, Kuma Shimizu, who is struggling to find employment, is suddenly transported to another world. He is taken to the women's prison, Sea Prison Castle, where he fuses with Kraken, the ruler of the sea, and gains great powers. Kraken entrusts Kuma with rescuing Princess Zeina, so he decides to sneak into Sea Prison Castle, but there he finds a group of female prisoners with unique personalities waiting for him...

(Source: Square Enix, translated)