Either the late 80s or early 90s I used to watched this movie. It is an animated movie that has a person from a waterworld and a girl from I guess from land or light based area. They are forbidden to be with each other. They end up falling in love and at the end they both die. Sorry it isn't more to go on but that is all I can remember.

Thanks

Sounds like this could be the Japanese animated movie Sea Prince and the Fire Child from 1981, the plot of which borrows from Romeo and Juliet.