Water is essentially incompressible, especially under normal conditions. If you fill a sandwich bag with water and put a straw into it, when you squeeze the baggie the water won't compress, but rather will shoot out the straw.

What is incompressible water?

Water is incompressible, which means that you can't squash it to make room for air. Air is compressible, which means that you can compress (or squash) the air and add a little bit more air. Think of two bottles: One bottle is entirely full of water – you can't blow air into this bottle.

Is anything truly incompressible?

For any finite force the particles can be squeezed together, and therefore the system can be compressed. No such infinite forces are known in nature, and therefore there is no such thing as a fundamentally incompressible system.