Well, using liquid CO2 would turn the whole installation into quite a complex thing.
In fact, the liquid phase, into this context, is only useful for storage.
When working pressure is required, liquid is turned into gas by means of the release valve, that makes pressure drop below the vapor tension of the liquid, at the working temperature (See Antoine's equation for aproximate calculation).
As it is gas and not liquid what is working, gas pressure must be equal to:
Working pressure+Ambient pressure.
It means that gas consumption will depend on depth.
A compressor+cooling unit is required for gas phase recovery, in order not to waste gass each time it's used.
Water is the obvious solution.
A centrifugal pump, is a device that is designed for rising pressure, and not for achieving a given one.
I mean that the pump will provide a fixed pressure difference (depending on the pump's curve), that will not depend on the sorrounding `pressure, but only on the pump's RPM.
Hence, it does not require any container vessel, neither any kind of regulating-recovery device.
Water is taken from the environment, pressure is dynamically raised, and once used, released to the environment again. Only an ECS for RPM control(It can be solved with fixed RPM if the pump is correctly choosen) and a release valve for pressure control, are needed.
The valve can be of the type "safety+manual opening".
It will open if the "OPEN" command is received, or if pressure rises over a given maximum "Safety Working Pressure".
Energy consumption would be almost constant.
If a high pressure differential is required (stronger grip capability), while holding the pump size, an alternative pump would make the work. But those devices are slower, energy consumption is higher, and their mechanics a lot more complex.