The Ultra Large Crude Carrier (ULCC) oil tanker Jahre Viking was the largest ship afloat in 2004. At 485.46 metres [1,503 feet long] from tip to tip, she surpasses the tallest land structure, the 424-meter Petronas tower of Malaysia.
It’s actually the world’s biggest man-made floating object (the “man-made” qualifier is important, I guess, when you’re competing with icebergs).