All materials respond to magnetism in either a diamagnetic or paramagnetic way.
Using a magnet and a boat we can demonstrate this diamagnetism. The results are the same regardless of the magnets polarity.
The example shown is done with a tube filled with water.
Having just the plastic works also, but the added material density of the water inside increases the effects slightly. You can even feel this repulsion if you hold the magnet in your hand.
You can use other items as a mast and get similar results, for example (wood, plastic, rubber) - Try it for yourself.
This is the real reason things are pushed towards earth and along with density and bouyancy there is no need for gravity. The earths magnetic field in the heavens is shown by Star Trails