How is it that ships made largely of steel can float when the density of steel is much higher than the density of water? Why is it possible to make oil tankers much larger than other ships?

Answer 1

it is not the density of steel one has to worry about, it is the overall density of the ship, which contains mostly air, wrapped in a steel hull.

Oil is bouyant, it is less dense than seawater, so it floats already.