What is airdraft?

What is the difference between a fully loaded oil tanker floating and an empty oil tanker floating?

The main difference is weight of course. When fully loaded, the ships airdraft is considerably reduced, as you can definitely see the ship is more 'in the water', or more immersed. When not loaded, or 'In ballast', the ship has several ballast tanks which are filled up with sea water in order to weigh the ship down a little, so as for it to not capsize with strong winds/rogue waves. And a ship's hull/bulbous bow, etc.. are designed to operate/navigate within a certain Loadline, hence the need to keep the ship immersed within certain parameters.