You cannot whistle underwater because water is too dense to vibrate like air.

Whistling is a physical phenomenon known as edge-tone production, which relies on the Helmholtz resonance principle. In the air, a person uses their tongue and lips to create a small aperture, forcing air out at a velocity that creates oscillating pressure waves. Because water is approximately 784 times denser than air at sea level, the human respiratory system cannot generate the force required to move such a heavy mass of fluid fast enough to produce these audible frequencies.Furthermore, water is nearly incompressible compared to air. In a gas, molecules can be squeezed together to create the high-pressure pulses that form sound waves, but in a liquid, this requires immense energy far beyond human capability. According to the Engineering ToolBox, the bulk modulus of water is about 2.15 gigapascals, meaning it resists compression with extreme force. This resistance prevents the rapid 'flutter' of the fluid stream necessary to initiate a whistle tone.The speed of sound also plays a critical role in this failure. Sound travels at roughly 343 meters per second in air but accelerates to approximately 1,481 meters per second in water. This drastic change shifts the resonant frequency of the human mouth cavity. Even if a person could move the water fast enough, the mouth would be the wrong size and shape to resonate at that speed, effectively silencing any attempt at a whistle.