Water is highly incompressible.
Now this is beyond my Physics abilities, but I'll sketch out what extreme pressure could do.
This means if you take water and reduce volume, pressure goes up and work is done, and the pressure will resist the volume reduction.
The work done by the reduced volume will generate a lot of heat.
If you continue to compress water, the energy requires diverges, the temperature diverges, and you end up with not-water. If you reduce the volume of water by a factor of 500, you get a density similar to a stellar core; the oxygen and hydrogen will long be no longer forming conventional molecular bonds (ie, not be water). I haven't done the math, but the temperature is likely to be high; the result is likely to be fusion. This will in turn generate even higher tempuratures.
Continued application of pressure without regard to how hard it is will compress the matter further. Electron degenerate matter has a density of 10^9 kg/m^3 or 10^6 g/cm^3, 1 million times more dense than water. Normally this occurs after most fusion has ended (because the heat of fusion prevents further collapse); as I am assuming your magic is absolute, you end up with heat values that are simply ridiculous, as fusion "inflation" isn't able to stop continued compression.
Compression at this point results in electrons not having states to shift into, so they merge with protons to form neutrons. These neutrons will be unable to decay, because the electron states are all occupied (that is what "electron degenerate" means).
Further compression will reach neutron star density, which is 10^8 denser.
Beyond that there is no known process that stops before a singularity occurs.
So taking 1 m^3 of water and making it 1 cm^3 is a 1,000,000x density increase, electron degeneracy levels.
Going to 1 mm^3 is another factor of 1000, which gives you some neutronium.
By 0.001 mm^3, neutron degeneracy fails, and I think you get a microscopic black hole. A black hole with a mass of 1 tonne has a temperature of 135315706004378016331 K. But that is far beyond my physics abilities.
For less absolute cases, we can work out what the pressure-volume relationship of water is.
The pressure is force per unit area, the surface area of the compressing chamber is the area, the distance is determined by how much the water compresses. This gives a force times distance, which tells us how much energy is involved.
Here is Dr Joule working on that exact problem, experimentally, 163 years ago.