The reason that an iceberg is able to cut into the side hull of a ship (e.g. Titanic etc.) or any other marine vessel is dual fold. The first reason is because steel becomes more brittle, the colder it gets. Steel created during the early 20th century was not tempered as steel is during the modern day. The second reason is because of inertia, weight, and force. An iceberg can slash through steel the same way a tree can rip through a vehicle that has hit it. An iceberg is a large, immovable object, so whether the hull is comprised of wood or titanium, the ice will always win out as it has more inertia and mass. To provide a frame of reference, bone is incredibly strong and tensile, however if a brick which is harmless while at rest, slams down from a height above the arm, the force it imparts is enough to break said bone