A clear blue sea under a beautiful blue sky.
After a long departure, the sea witch Wadanohara travels back home with her familiars.
But her ex-familiar, the shark Samekichi, appears to block her way:
"You shouldn't be here... Leave this sea, right now."

WARNING:
Deep-Sea Prisoner rates this game "ages 15 and up," mostly due to graphic content later in the game.
(This rating is Japan-centric and would likely be more strict by other countries' standards, 18 and up may be more appropriate.)
I would advise against playing if you think you may be upset by implied sexual assault and violent imagery.
Deep-Sea Prisoner also insists that players should be able to keep reality and fiction separate.