Sango Shou Densetsu: Aoi Umi no Elfie

I have to say that I watched this when I was 6 to 7 years old and I never forgot it. This anime touched me more than many Disney movies when I was a child. I kept searching for it but had no idea where to look and what keywords exactly to look up. Of course after all this time (me being 26 now) I couldn't remember exactly how it was.
And here by chance I just found it. So happy about this! I kept remembering the final scene and couldn't let go of it.

I think this anime deserves a remake to renew the graphics, ... sound quality, etc

This may contain a few spoilers so I'm warning you.

The first half of this movie (actually more than half of it) was focusing more on the describing the "world she lives in and what the humans are doing wrong (the damage on the damage on the sea) and throughout this part we slowly get to understand more about what exactly she is, even if you can easily guess after the first scene where we see that.
But then this revelation is a problem in the 2nd half because of the way others will see it and eventually use against her and we get to a part ... where she is taken and is shown to the public and I felt like this scene had little to no impact.
Overall this actually wasn't a bad movie, even tho it was rather straight-forward and with an optimistic ending and the major problem here seemed rather easily solved because it didn't take long. Even if this movie could have been much better but I still enjoyed it and would recommend to someone interested.

Sure, this may be a fairly simplistic story, with unimpressive artwork and less-than-stellar animation, and it may be heavy-handed on the moralizing, aimed at kids in the eight to ten year age range. But overall...not that bad.

It's hundreds of years in the future, and Earth has turned into a Waterworld, mostly ocean and humans living more in harmony with nature (out of necessity). Elfie, with her brother and grandfather live a seemingly peaceful life, flitting about (and under) the seas in futuristic ski-doos, playing with the dolphins and worrying about the growing encroachment of their nation's development on the ... pristine seas.

Unexpectedly Elfie discovers she can breathe under water, and soon becomes hopelessly entangled in the middle of a conflict between the humans above the sea (led by Ming the Merciless) and the mythical and much-feared undersea folk.

It's not a very sophisticated story, which is fine, not every story has to have some big twist or multi-faceted relationship polygon. The ending is somewhat abrupt and deus-ex-machina in nature, but oh well. Good story for the younger set, and their adult parents will probably be okay with it, too.