I’m always amazed at the random and obscure knowledge found on these pages; so here’s another test!
I believe this is a TV movie- an “ABC Friday Night Movie” thing, from the 70’s. I think it was based on some sort of old legend of some sort, taking place in and around the ocean (pacific/south pacific). The heroine is a teenage girl, and I think there might have been a boy also (no, it’s not “The Blue Lagoon”!).
And here’s the final, a possibly most important clue: there’s a giant sea turtle in the story. I can’t remember how giant, maybe bigger than a schoolbus? And I think the movie ends with the turtle swimming off into the ocean, never to be seen again…
No claim to this being it, but there was one called “Turtle Diary” that had (I believe) Tom Conti and some other English actress. Now I’ll go IMDBing for at least the page where you can read about it.
It wasn’t “Turtle Diary” since that was 1985. And it wasn’t Tom Conti, but Ben Kingsley. And it was Glenda Jackson. All round proof that I have no idea.