It's important to know that radiation isn't one thing.

The three main types that people mean when they use that term are alpha, beta, and gamma.

Alpha particles are little helium nuclei. They can mess up DNA (maybe causing cancer), but they don't travel far and are usually only harmful if the source is inside you.

Beta particles are electrons moving at incredible speed. Not as likely to cause damage as alpha particles, but they travel much further and can cause damage if you are near the emitting material.

Gamma radiation is just very energetic photons (light). Gamma rays are the least likely to do damage because they will likely pass straight through you without interacting. They travel incredible distances though and are difficult to shield against.

X-rays are the same kind of thing as gamma rays, but they have less energy. They are more likely to interact with your body and are more easily blocked. They can do some damage, but the probability is relatively low. Prolonged exposure is bad, but they are low risk for occasional medical imaging.

It is important to note that all of these are harmful because they are ionizing radiation. They can damage complex molecules needed by the body (mostly DNA but also cell structure if in sufficient quantity).

Water is a very stable molecule and x-rays aren't powerful enough to change it. You can actually sterilize water by irradiating it with UV light or stronger (UV is like x-rays but even lower energy), because the molecules that make up bacteria and viruses are more fragile than water molecules.