Dry dock vessels can sink and float

Image: U.S. Navy, YouTube

Right under the Coronado Bridge is California's largest floating drydock. This vessel, christened Pride of California, purposefully floods itself and sinks low enough so a ship can sail inside. The dry dock can then resurface, bringing the ship inside it out of the water for repairs.

How can a vessel sink and float on command? What causes a vessel to float? Why do some objects sink and some float?

Resources:

YouTube: USS Montgomery Enters Drydock