Water measuring is a way of precisely measuring the volume of specific parts of your gun. This is mostly needed for calculating the C:B ratio or needed for calculating the amount of fuel needed to be injected by a fuel meter. The volume of cylindric shapes can easily be calculated, but the volume of an elbow or other fittings is ridicilously hard to calculate. Therefore you can use this method.
You fill up the part you want to measure (your chamber + elbows for example) with water. Then you take the water out bit by bit and measure the volume of it using a measuring cup. When you got all the water out, add up all volumes you measured and voila, you got the volume of your chamber.
PVC is made to carry water so this won't harm your cannon, but ofcourse you should take stuff like your chamber fan out. Some types of metal, like ungalvanised steel may rust, and thus you could consider using oil.