All liquids have a boiling point, which is determined by a combination of temperature and pressure. You can boil water either by heating it to 100º C at sea level or by keeping it at room temperature, placing in in a vacuum chamber and pulling the pressure down to the boiling point!
However, that's a lousy way to make coffee.
When a propeller is rotating, there are areas of high pressure, where in effect the prop is pushing water away from it, and areas of lower pressure, where it is sucking the water toward it. Especially at the surface, where water pressure isn't too high to begin with, it is possible to spin the prop fast enough that on the forward side of the prop (the low pressure side) the water actually boils. These bubbles, of course get sucked into the prop, which because of reduced drag from the bubbles, spins up without producing thrust in the bargain. It's like spinning your tires on sand.
That's bad enough, but believe it or not, the formation of these bubbles is a very violent, if small act! They actually erode the surface of the propeller. Since cavitation generally starts out at surface irregularities such as dings to begin with, you can see that as cavitation damages the prop it causes....more cavitation!
It feeds on itself, making your boat slow, causing tons of noise for enemy sonar operators to hear, killing your fuel economy, destroying your propeller, it's just a bad scene.