Sea Prince and the Fire Child

I saw this on either HBO or Showtime in the early 80's (As the legend of Sirius) and was entranced by it. The story is a retelling of Romeo and Juliette, though with a happier ending (Not a difficult feat). It also reminds me greatly (especially Ursula) of Disney's 'The Little Mermaid', though that came much later (And wasn't as good). The final battle between Posideon and 'Ursula' (A very different beast that I can't remember the actual name of) is better than Disney's offering, including the animation.

Our hero Sirius, a water child, falls in love with Marta, a fire child. In the english translation, they were Prince and Princess, respectively though that doesn't affect the storyline much. Friends are lost, and other sacrifices are made in the name of true love, then....well, can't give away the ending.