– Why is water ‘virtually’ incompressible?
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It depends on the context. At Earth’s surface at about one atmosphere of pressure, you can put air in a canister and push on it pretty easily to squeeze it and increase its pressure. For a liquid, you need very high pressures (and thus strong/stiff containers) and pretty sensitive measuring devices to detect it.
https://www.usgs.gov/special-topics/water-science-school/science/water-compressibility says that 150x atmospheric pressure water will compress by 1%. That same pressure would compress a gas to less than 1% of its volume at the surface.
“Virtually” comes from being able to say that the effect is small enough that it can be ignored to make the calculations easier. How much gravity is there between your body and your neighbor? It can be calculated, but it is so tiny relative to the gravity between your body and Earth, so it can be ignored.
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