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Gary-
 
 Right on the first half. But consider this: What happens to a diver when they 
go deeper in the water? The *diver* is under more pressure, and will be 
eventually crushed from it. The fact that the water is incompressible has 
nothing to do with the fact that the water is under more pressure as it gets 
deeper.

 Because the deeper water is under more pressure (yet not compressed) means 
that the edges of the prop are under more pressure, and because the deeper 
parts are under more pressure the air does not readily "boil" off the bottom 
half of the blade, even while it is boiling off the top--cavitating.

 Some interesting experiments with water pressure: Take a dime store snorkel, 
use it. Now buy on of those 4-foot long models, and TRY to use it. The 
pressure difference of the extra three feet makes it nearly impossible for 
your chest muscles to bring in fresh air and use it. In fact, in SCUBA class 
you would be taught never to come upright (from a horizontal position) 
without exhaling, because the air in you lungs "down there" can expand 
sufficiently to explode a lung only 4' higher up, when you "stand".

 The water doesn't have to be compressed, in oder to transmit pressure. That's 
one reason hydraulic systems work so well.

 Incidentally, as you go deeper the water tends to be colder. Colder water can 
hold more dissolved gasses than warmer water--which is why ice cubes made 
from boiled water always have fewer bubbles than ice cubes made from tap 
water. During the boiling process, the water can't hold the gasses and they 
are driven off. Surface water in an ocean also will be able to hold less 
dissolved gas than water even a foot or three down.

 This is why old tires migrate to the top of landfills. Literally--the tires 
are "flowing" to the lower pressure at the top of the pile, where nothing 
else is pressing down on them.

 Ain't magic, just odd science.

But I still say bouyancy doesn't float boats, Poseidon's favor does.

   

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