Simply and rather surprisingly, it is the water boiling on the suction side of the propeller blade. The suction side of the propeller is the side of the propeller facing the direction of movement of the boat. This happens when the pressure on the suction side is reduced to such an extent that the boiling point of water is reduced to the ambient temperature of the water. Remember that the boiling point of water is a function of pressure and temperature. They have a direct relationship. Lower the pressure, and the boiling point is lowered as well. Bubbles are formed when the pressure on the suction side of the propeller becomes less than the ambient vapor pressure. The resultant bubbles move towards the area of high pressure, where they eventually burst, which causes cavities on the blade. For a more intense explanation, have a look at the following site: (https://www.theshipyardblog.com/propeller-cavitation-explained/)