Also, the legend of the mermaid princess sounds conveniently foreshadowing of events soon to occur in fishman island. In fact, I think it ties well with a few other details. Theory time!

Luffy is the man foretold that will appear as a guide to Shirahoshi, and will change fishman island forever.
This will result in the destruction of fishman island, as foretold by madame whats-her-name (Sea Kings decimating the island).
However, ultimately it will open up the path for true coexistence, as the fishmen will subsequently migrate to the surface, and Shirahoshi will be their new leader.