The Incompressibility of Water
The reason water, and many liquids, are so incompressible is that the atoms or molecues are about the same distance apart as if the water were frozen to make ice. When you try to compress water the molecules are forced into proximity and repel each other strongly. Uniquely, when water freezes the distance between molecules increases slightly. The molecules in a gas are about times further apart.
In fact water usually contains dissolved gases. When we compress water, we are usually compressing the dissolved gas in the water.