Need help remembering an old cartoon

Okay, this will sound very strange, but please bear with me. I am trying to remember an old cartoon---I believe it was an animated movie. I think it *may* have been on Nickelodeon, but it might have been Disney too, not sure.

I can't describe much; I was a little kid. All I remember is a little boy, floating underwater, following some kind of guide i think? Lots of white, beige, lots of crystals everywhere.

I already found one of the old movies I couldn't quite remember...Unico. So I am hoping to find the one I described above, too.

You may not remember it if you weren't an 80's kid. I want to say I saw it around the era between '84 and '89.

Any help would be SO appreciated.