@Maju:

you have not a clear idea of how onepiece world geography is..we are miles away from the red line at the moment,and fishman island is located exactly below that
also,he is a devil fruit user..he would have drowned before reaching the island

Well the marines were sailing away from Dressrosa toward Impel Down or the Gates of Justice. Dressrosa is fairly close to Punk Hazard, like a days travel. Punk hazard was right beside the red line. I am fairly sure we could see both the island and the red line when the straw hats rose.

Also we do not know the capabilities of Jacks ship. He could have been picked up underwater. He had to have been picked up somewhere if he survived, and obviously it was not in sight of the marines.

Possibly he could have even been hit by a gravity technique and shot underwater.

I had a dream about One Piece last night, and I had a theory that I found interesting enough to create an account to post. Hopefully I am not repeating someone else's theory.

I think that Jacks body was not found because he went down under water and ended up on fish man island. Now he is there crucifying Mermen, maybe Shirihoshi and her family. Sanji might have went there to save them, and took Caesar with him in case there were gas weapons. This leads to Luffy going back and destroying the island.