Granted, as Chapter 610 reveals, Sharley foresees a human in a straw hat who will bring ruin to Fishman Island, though it is yet to be confirmed whether this individual is in fact Luffy.

Given that this is a Mythical Zoan, Luffy's fighting powers are boosted tremendously using this devil fruit, provided he uses ... terms of Haki. As was pointed out in the Egghead Island Arc ...