There was a movie I use to watch when I was like, 5, around 1988 or so. I have no idea about the name or much about the movie but I know I just adored it.
All I can remember is that it was about a young boy who lived underwater, but not a merman/mermaid. I think he met a young girl who lived in another place, I think possibly land and wanted to stay with her. For some reason I think he had to give up something to do that though. I think his dad may have been the king or something and he was a prince and he couldn't come back if he left.
I know that is SO detailed, but if someone has ANY idea, I would love to hear it. Thanks! :confused:

It's called Sea Prince and the Fire Child

Shiriusu no densetsu (1981) (http://www.imdb.com/title/tt0203895/)

Oh my gosh! You just made my morning!! Thank you so much! That was totally it! :D