As soon as you start to make a vacuum, the water will start to bil until the vapor pressure equals the vapor pressure of the water at whatever temperature it's being kept. This will continue as you slowly expand the volume until all the water has turned into gas. Further expansion will leave a lower-density gas filling the space.
If you pull the piston out quickly, the rapid evaporation will cool the water temporarily, until heat can flow in from the surroundings.
On another issue, no material si incompressible. That's just an approximation useful for some purposes.
Mike W.
(published on 07/21/2020)
