There are many things needed to be considered to answer this.
- First is the unfulfilled promise of Joy Boy to Poseidon, this promise includes transportation thus they created the Ark Noah. Ark big enough to carry all Fishmen. The Fishmen kept on protecting the Noah believing that one day Joy Boy would fulfill this. In my speculation this Joy Boy is actually one of "D." Luffy's ancestor, this would make sense why Roger and Luffy was able somehow to hear Sea Kings. Leaving Luffy to fulfill this.
- Second is the fact that Luffy accepted Fukaboshi's wish to "bring them to Zero" or surface of the sea because they're friends, as how Luffy said.
- Third is the fact that the Straw Hats made a pinky promise to show Shirahoshi around the surface to see the real forest, racoons, etc.
All these facts "may" lead Luffy and his crew to bring the Fishmen to the surface and this could be the very reason for Luffy to destroy the Fishmen Island for some reason like the fishmen might refuse to leave their island leaving Luffy no choice but destroy it would be a possibility. Because no matter how I look at it an island kept at the dark bottom of the sea isn't something nice, something Luffy might think himself. But since the species of Fishmen are discriminated by the world, Luffy requires power before doing this to carry and protect the species under his name like how Whitebeard protected the island as a Yonko.
As for how to destroy the island. You could come up with many possibilies like maybe popping the bubble that is keeping it afloat, or use force to the center of the island enough to destroy it, or somehow talk to the Sea Kings, or a fierce battle, etc...