Can somenody tell me why if water cannot be compressed, the water pressure at ten miles underwater is several hundred times the pressure when only 10 feet underwater?

Just curious.

It is the weight of the water above that causes the pressure, not the water being compressed. Same as it is the weight of the air in the atmosphere that gives us the sea level pressure of one atmosphere. You are probably thinking that "yeah but the atmosphere is thinner up high so the pressure is different", well that's true but the atmosphere is a gas and the molecules are such that in a gas they can be compressed, there's more "space" between them than what there is in water.

I'm not a hydraulisist BTW.:)

Actually, fluids are compressable to a minute degree. A thing I have to take into account during work. During the proving of fluid meters, called calibration, a 4 digit factor is used to allow for the compressability of the fluid being measured. Dependant upon both gravity and visocity of the measured fluid.

ahhh ok! thanks

Thus does arise another question. How does one determine the surface of a gas giant planet?

An atmosphere is a medium whose density increases with depth (gaseous). An ocean is a medium whose density remains essentially contant (liquid). The transition boundry is the surface.

You may ahve noticed a small device called a barometre. It measures the compression of a fluid, namely air, which is caused by the weight of all the other air in our gravity well. It can vary greatly. The weight of all the material pressing in due to gravity is compression. The further down into the gravity well you go, the greater the compression, and thus the greater the pressure. There are indeed different pressres (meaning different compressions of the material) in the coeans. This is why we have an effect/phenomena sometimes called the thermocline. Due to this effect/phenomena, signals through the ocean can be bent, redirected, stopped, and generally interfered with. It's one method submariners use for hiding from ships. Get your submarines hidden under a change of temperature/pressure layers, and you have a better chance of hiding from a sonar signal. So yes, the further down you go, the greater the pressure.

I almost forgot to mention... Pressure checkers worn by divers and in submarines, the Bends, ever heard of them? They exist because the pressure increases the further down you go.

Got a few things scrambled:

Thermoclines: Are not a result of pressure but of temperature;

Submarines...: Use 'passive' rather than 'active' sonar most of the time.
Surface ASW only effective in litorial waters due to the great depth
modern subs operate at;

And, bends......: Not experienced by submariners. Normally at close to
atmospheric pressue internally. Only additional air bled into the vessel
was at the beginning of the dive to test hull integrety (all valves
closed). Don't know if it applies to nukes, only served aboard diesels.

Finally, diving.....: Time also a factor. A single tank (72 cf) dive to 180 ft.
only requires a slow ascent due to the short time spent 'at the bottom'
whereas a multi-tank or multi-dives to 60 ft. require a minimum of 5 min.
@15 ft. (if I remember my dive tables correctly).

Take care ;)