[contravariant]

I reckon that water under high pressure does have a somewhat higher density. However water is pretty much the textbook example of an incompressible fluid so the difference will probably be rather small.

Edit: Found a direct quote on wikipedia https://en.wikipedia.org/wiki/Properties_of_water#Compressib...: >The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.[40]

[jacquesm]

Water is tremendously stable, cheap and commonly available, that's exactly why it - and nowadays oil - was used since time immemorial as a working fluid to transfer force from one place to another. In essence the limiting factor is the burst strength of the tubing.