Whose principle explains why objects feel lighter in water?

This is Archimedes' principle, named for the Greek mathematician and inventor Archimedes of Syracuse, who lived in the 3rd century BCE. It explains buoyancy in liquids and gases and is the reason ships float and hot-air balloons rise. The idea is one of the foundations of fluid mechanics and hydrostatics. Archimedes also became famous for work on levers, pulleys, and geometry. His name survives in one of the most important laws in physics.