Squeeze the bottle and the diver sinks. Let go and it surfaces.
When the bottle is squeezed, the air compresses as water is nearly incompressible. The air bubble in the diver shrinks allowing more water flow inside and increasing the density of the diver. By Archimedes' principle the buoyant force on the diver remains constant, so the diver falls to the bottom. When you let go, the process is reversed.
The paint on individual divers may vary.
Dimensions: diver is about 3" (7.6 cm) in height.