That's very interesting indeed. Cavitation, the formation of bubbles in a liquid, typically by the movement of a propeller through it, creates heat as well as light. That's why propeller blades get worn down on a boat and have to be replaced. Friction is part of the reason, of course, but research conducted by the Navy measured temperatures of 10,000 degrees F that lasting nanoseconds on the edges of rotating propeller blades.

One man had created a fuelless, on demand water heater that heats water using this cavitation effect. It runs on a small motor and creates incredible amounts of hot water.