To understand the dangers of radioactivity in water, you must be able to distinguish between radiation and radioactivity.
Radiation is energy being emitted by some kind of particle. When particles that have mass emit radiation, they are said to be radioactive. For example, sunshine is a form of radiation. The sun, from which the sunshine is emitted, is a massive object that is radioactive.
The water molecule itself, is not and cannot be made to be radioactive (emit radiation.) When radiation hits water, it splits the water molecule into Hydrogen (H+) and a hydroxyl (OH-) radical, neither of which are radioactive. However, these ions can be dangerous because they can cause damage to cell structures including DNA.
The drawing to the right shows how radiation (top arrow) splits a water molecule to produce a hydrogen atom and a hydroxyl radical, and how that hydroxyl radical can affect DNA in a cell.
Because water is such an excellent solvent, water droplets tend to pick up gasses and particles carried by winds aloft which then fall to the ground as rain. Sooner or later this rain may become part of the water supply used for human consumption or watering crops. Water that comes from deep underground aquifers may also contain radionuclides (radioactive atoms or molecules) including radon gas, alpha and beta particles, radium and uranium.