Umi no Triton

My mom plays Pipi in this. The series was made before I was born (1973). She also sang the ending credits theme song I think.

Green-haired orphan Toriton and his sassy mermaid friend Pipi are the 2 surviving descendants of Atlantis, the mythical Greek island that sank during an earthquake.

Toriton washes up on the shore and is raised by a kindly fisherman. He has superpowers underwater, but kids tease him about his green hair and what not. One day he discovers a glowing red dagger stashed away in the closet. Also a conch shell that contains a pre-recorded message from him parents, explaining that they had to send him off to sea before Atlantis sank.

They team up with a wise, matronly dolphin, meet a Yoda-like giant turtle who dies, and then go fight Poseidon (=Neptune in Roman mythology) and his evil minions with his red dagger.

Very much like Star Wars and Superman, plus Greek mythology stuff thrown in

I guess this is not the place, but the date on this series must be wrong; that is, of course, if we are talking about the same thing. I don't know exactly what year, but I must have seen this show before 1976. In any case this series left an indelible mark on me. It is a captivating story about a boy that lives on a beach and one day gets the call of his true nature. A group of dolphins come to him to deliver the message: he's in reality the savior of the seas, that have been taken over by some evil forces. It's somehow a mixture of Greek mythology references loosely related to Neptune and his kin; or so I remember. He embarks on a long journey, full of anticipation and anguish, as every good Japanese animated feature usually has, or at least, used to have. The kid has a fire sword and plenty of good and evil characters to befriend or fight. The idea was to get Poseidon, the great king of evil, who is the head of all this forces that have taken control; and restore peace and freedom to the seas. I specially remember a gigantic turtle who was like the epitome of good; he appeared only on the first episode though.