Well, I have many things to say. First is about Shirahoshi. Other than being the biggest and most beautiful mermaid, she is unique in many ways. But how will Shirahoshi feel if she learns that she is an ancient weapon? After all, she is a kind and gentle person. Some of you started thinking that Shirahoshi might become a Straw Hat member. Yes, she has an ability to summon Sea Kings, which can bring devastation to the Straw Hats' enemies. But answer this, when has Shirahoshi ever shown a desire to hurt others? She could have turned Hody Jones in after learning that he was the one who killed her mother, but didn't. Anyway, her role in this story will be much clear later on.
Next is about Shirley's prediction. She predicted that Luffy will destroy Fishman Island. But the ones who tried to destroy the island were Hody and Decken. Luffy just saved the island so either Shirley's prediction was wrong or misintrepreted. However, Shirley did say that what she forsaw can happen one year later. So I guess that if Fishman Island is going to be destroyed, it might happen during the "war that engulfs the world", the war that Whitebeard said will happen. Anyway, that also might become clear later on.
As for the officers of the New Fishman Pirates and Vander Decken IX, they all share the same cell together. Let's assume they wake up and Decken finds himself all alone with Hody and his officers. If Hody and Decken still hate and want to kill each other, then it'll be safe to assume that Decken won't be thinking about Shirahoshi for a long time, that is if Hody doesn't tear Decken to shreds first. After all, I highly doubt that Decken is allowed to keep his weapons and we already saw how easily Hody crushed Decken back at Noah. Plus, the officers, save for Hyouzou, is loyal to Hody to the end, so the odds are not in Decken's favor. Anyway, Hody and Decken fighting each other in their cell is what I would like to see.