no matter how deep the prop, the water on the lower side
is still going to be under ambient higher pressure than the water on the
top.

the pressure difference due to water depth is inconsequential. The pressure
change is less than 0.5 pound per foot of depth. the compressibility of water
is near zero. even on a 15" prop, the center of effort difference between top
and bottom blades is less than about a foot. These ar truly miniscule forces
as compared to the force needed to move a multi-thousand pound boat in a
noticeable fashion.