When you compress liquid water together, the molecular forces become very strong stopping it from being compressed very much. However, for a gas the molecules are so far apart that the forces are much smaller (the main reason a gas can't be compressed is due to the kinetic energy of the molecules in the gas).

How can water be compressed?

Pressure and temperature can affect compressibility

But, squeeze hard enough and water will compress—shrink in size and become more dense ... but not by very much. Envision the water a mile deep in the ocean. ... Even with this much pressure, water only compresses less than one percent.

Why can water not be compressed?

All these things are possible because water is difficult to compress – the molecules attract each other and, in their natural state, tend to stay closer together than the molecules in other liquids. The harder something is to compress, the easier it is to move it around if you apply a pressure to one side of it.