Blood is thicker than water...
But still thinner than air...
All the blackout/redout issues with fighter pilots are due to centrifugal (tidal) forces with internal water-based fluids vs external gas.
Substitute fluid for air, and yoiu've fixed one variable.
That is, until you reach the accelaration where you centrifuge your kidneys away from the rest of your body.
Next-Level-Problem :-
Caisson Disease - bone dissolution under extreme pressure (read about "carbonate compensation depth" in the Pacific Ocean!)