Eli5: If water can’t be compressed under normal conditions, then how does water pressure work?
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Others have already answered the question: water _is_ compressible, and yet this is mostly unrelated to pressure.
But let me give you a very simple proof that water can be compressed: it conducts sound as everyone can test in a bathtub or lake. Sound is produced by small differences in pressure that propagate as a wave. Anything that cannot be compressed cannot conduct sound. And indeed, even other types of compression such as pushing on a stick or pumping water into one end of a pipe are moving through the system at its speed of sound!
However, there are no proper substances that truly are incompressible; as more abstract examples, vacuum and black holes could maybe be used, both of which indeed do not compress nor conduct sound.
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