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]

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.

Yes, I just wanted to correct OP statement that "and have more water above you, causing more downward pressure" by pointing that increasing pressure exerts force on bottom side, so in effect it cancels out increasing pressure pushing at top, and so buoyant force is constant. (with added trivia about minuscule changes due to fluid compressibility)

Hmm, I think have gotten my physics backwards with regards to the correlation of more pressure to more downward force.

Thank you for correcting this.

Water density increases with depth. Though only 4.96% at the bottom of the Mariana Trench.

Near the surface it’s almost never an issue, but for ultra deep dives it’s important.