Archimedes’ principle tells us that the water displaced by a ship weighs as much as the ship itself. If we can figure out the total weight of all the world’s ships, we can figure out how much water they’re displacing, then divide that volume by the surface area of the ocean to figure out how much the water level would drop.
The worlds shipping fleet (at the time of this article) weighs about 2.15 billion tons when fully loaded. The main component of the fleet by weight is oil tankers and bulk ore-carrying ships, which make up over two-thirds of the total.
To find out the answer – follow this link: https://what-if.xkcd.com/33/