Derek,
I agree that atmospheric pressure reduces with height above sea-level but I cannot agree that water will be forced from the lower point to the higher one.
What you are suggesting means we could use a hollow mast, open at its foot to the water, and let 'scientific fact' cause the mast to fill with water? From where it could flow back down, by gravity, giving us a perpetual motion?
You know as well as I do, you are mixing-up the 'scientific' properties of two materials, water and air: take into account the vastly different specific gravities of the two, the water will attempt to maintain a constant surface-level. It would be a bit embarrassing if hydro-electric schemes found their water was flowing back up the mountain because
"water will always be forced from the lower point to the higher point" It does however flow from the
higher pressure (headstock) pipe, across the turbines, to the
lower pressure (tail) pipe where the
pressures are both water-pressure not air pressure.
I think what you are actually trying to say is that water will flow from the (relatively) high
water pressure in the tube to the
air pressure inside the hull: water pressure increases (about) .15 psi per foot of depth so, if the inboard end of the prop-tube is two inches below water-line the water in it is at a pressure 1.005 atm (=14.7700psi) compared with inside the hull, which is at 1 atm (14.6959 psi) (BTW, I used this tool to get these numbers:
http://www.calctool.org/CALC/other/games/depth_press)
Geoff