Accordingly, there are 10,377 ships in all the world's armies, 53,732 ships in all the world's merchant fleets, approximately 4.6 million fishing boats worldwide, and roughly 30 million recreational vessels. This therefore amounts to approximately 35 million ships of all classes.
The largest ship ever built in the world was the Seawise Giant , which displaces 657,019 tons.
If the 35 million ships on Earth were that large, they would displace about 23 gigatons (23 x 10e⁹). In contrast, all the water in Earth's oceans weighs on the order of 1.4 exatons (1.4 x 10e¹⁸). That's a difference of 9 orders of magnitude.
The weight of 23 gigatons, therefore, translates well to a volume of 23 cubic kilometers. The area of Earth's oceans is 361.9 million square kilometers. If we spread those 23 cubic kilometers over the total area of the oceans (23/361,900,000), we find that the ocean would shrink by about 0.064 millimeters (approximately the thickness of a sheet of paper).