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It's not incompressible it's just that when you compress it, it will stay liquid and it's volume doesn't really decrease that much as the chart below shows.

http://engineering-references.sbainvent.com/thermodynamics/p...

It's only like ten times more compressible than diamond, if I remember my schooling correctly.

In liquid water, mechanical engineers are usually free to assume incompressability. This makes solving the math a lot easier, though absolutely cannot be done for gases.

Well, even for gasses, your results are accurate enough as long as the Mach number is below 0.3: http://encyclopedia2.thefreedictionary.com/incompressible+fl...

This is not done in practice. Be careful getting engineering advice from the Free Dictionary.

It reflected what I was taught in fluid/gas dynamics courses for my degree, and you'll see the same point made in various other places.

"The compressibility of water reduces the sea level by about 40 m giving us 5% more land."

http://www.starfish.ch/reef/ocean.html

Nothing is truly incompressible.

Right, if something were truly incompressible, the speed of sound through it would be infinite and you could propagate signals through it faster than allowed by relativity.