During the Fishman Island Arc, Madam Shyarly's in Coral hill predicted that a man with a straw hat is destined to destroy the Fishman Island.
They say Madam Shyarly's prediction never failed. I can't think of any other besides Luffy that is wearing that hat.
How can Luffy possibly do it? Because in the first place, he saved that island and he's not that kind of guy who will do such a thing. Also, Fishman island lies right underneath the Red Line and Mariejois. So destroying Fishman island means destroying the capital of World Government, Mariejois.
How can Luffy possibly do it? (if he is the destined one because it lies beneath Mariejois and Red Line)
Could this be another straw hat guy? or her prediction is just wrong?
Strictly speaking there are at least 4 possibilities which are all possible:
Luffy's actions could bring about the destruction of Fishman Island due to his risky behavior. For instance: he claimed it as his own and challenged Big Mom for it so she could destroy it and it would be Luffy's fault.
Someone could destroy it impersonating Luffy in order to turn his rapidly growing group of allies against him.
Luffy could change the island so much that it could seem like destruction. He could, for instance, bring it to the surface or promote desegragation of the world so an island inhabited by just Fishmen/Merfolk is no longer a thing.
Finally, Luffy could be forced to destroy it for strategic reasons such as might be required in the "alter the Red Line so there is just one Giant Ocean and the world can be easily transversed" fan theories.
Ulitmately though, we don't know what will happened and any theories are speculation. The residents of Fishman Island have been convinced that Luffy will do them no harm, even though that is foolish, as Shyarly's prediction are reportedly never wrong. We can assume from his character, however, that he will not intentionally do anything to harm Fishman Island without suitable recompense. We can also assume, however, that this important plot point will not be forgotten and Oda will provide an explanation in the plot at a later date.
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...
Luffy does not destroy Fishman Island. However, it is a possibility that it could happen considering that the islands of Raftel (where One Piece is located supposedly) and Mariejois (the tenryubito homeland) are possible arcs that could bring about a calamity strong enough to make that happen.
She (Madam Shyarly) is also very accurate in her predictions as she predicted the Great Age of Pirates as well as Whitebeard's death. She recently made a prediction that a man in a straw hat, who she presumes to be Luffy, will destroy Fishman Island. However, Luffy left Fishman Island without destroying it so that prediction has not come to pass. But since events that Shyarly foresaw can happen anytime from a day to a year later, there is still a possibility that Shyarly's prediction is not wrong yet. By breaking her own crystal ball, Shyarly has given up on her scrying abilities.
I used spoiler tag because I think you are not finished to watch or read until the end of Fishman Island Arc.
Since Madam Shyarly predicted that a man with a straw hat would destroy fishman island in the future, it cannot be ascertained from this statement only what would be the state of fishman island or the world when this would happen. Assuming the man was straw hat Luffy, it still opens up endless possibilities. One of them could be and I personally prefer this one, the fishman island is taken over by an enemy, either the world government or a yonko and killed all the fishermen who were there and weren't able to escape. Doing so has shifted the balance in power and if something isn't done immediately, the world would come to ruin. This could be during the great war which might happen at the end or before that.
These all are just theories and it all finally depends on Oda and well Oda never takes the obvious way out.
I don't think with the introduction of DF awakening and seeing how strong Pica, or whitebeard were, it's farfetched to say that a single individual can destroy an island.
Here's my speculation on why Luffy will destroy fish men island.
It could be because that fish men no longer has the need to live under water and they can live amongst humans so luffy simply destroyed fish men island as it is not needed.