i expected it to vaporise or cause a nuclear fusion (not really, but that's maybe what would happen if pressed hard enough without it being able to escape early or escape at all)
You're not wrong in theory.. at about 3Gpa water becomes a special form of ice known as Ice VII. I am no expert.. but I imagine generating that pressure with this kinda setup would cause the container to fail first unless it was specially designed to withstand it. For reference 3 GPa for is about 30,000 times normal armospheric pressure. You would get to that pressure at water depth of about 300 km.. or about 30 times deeper than the deepest point in the ocean.. oh also temperature is obviously important too but apparently ice vii is stable accross a wide range of temperatures ( with variation in the pressure but same order of magnitude)