I searched through the Magnum P.I. episode guide but could not find the episode I was looking for. I'm not even sure which season it was. It could have been any of seasons 1 through 5 I think. I saw the episode around 1985 (roughly) I think, but I don't know if it was a rerun or not. It was shown during the normal time slot at the time.
All I can remember is that Magnum was stranded on an island or something, and he was with a woman in a black dress. At the end they were both rescued by a helicopter.