[James98]

The mercury tube is just old history to explain how these measurements were created. Nobody actually uses that type of gauge any more. If you used water, you are right in that evaporation would not allow a perfect vacuum, but theoretically the column would be 32 ft high. Mercury is a liquid metal and is the densest liquid known which is why it was used to measure very low pressures. And after you turn the tube upside down the vacuum is at the  top, the closed end.

Mercury was also used in early thermometers because it wouldn't freeze or boil.