Giant cargo boat that is flat in the middle, often anchored near the Verrazano Bridge in Brooklyn.?

I believe the ship you are talking about is used to carry very tall cargo and still be able to pass under the bridge. I have seen a ship there that was carrying very tall cranes of the type used to unload ships. A normal height ship would not pass under the bridge with those cranes. Some of those ships can actually lower themselves deeper into the water by flooding and have floating cargo (other ships or oil rigs, etc) float over the cargo area into position and then the ship will pump out the water and rise under the cargo lifting it free of the water.