I'm currently working on a graphic novel in which I need a cryptic sea chart.

In the story, the hero has discovered an ancient sea chart that shows the way to a mysterious, uninhabited island outside the Phillipines.

This may be a hard nut to crack, I want the chart to be cryptic and decorative, it shouldn't look like an ordinary map, I'm thinking it has to be based on some other system of navigation (the moon and the stars? but how?).

I don't need an actual map, just ideas how I coould make it work, the map/chart doesn't even have to be complicated, just DIFFERENT.

Any ideas? Help would be much appreciated!