Randall Munroe, the cartoonist behind xkcd and a profoundly brainy guy, answers bizarre physics questions submitted to his What If? blog. His most recent question pondered the effect of the sudden absence of the water displacement of all of the world's oceangoing vessels:
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.
And his answer to the question? The sea level would drop 6 microns. But because the world's sea level is rising anyway, it would return to the previous level within 16 hours.