In most cases it is not necessary to test for radon in water. Radon in water is very rare in municipal water sources. It is sometimes found in well water sources.
The primary health risk of Radon in Water is when the radon escapes the water and enters the air in your home. This becomes radon gas that you can breathe, which can damage your lungs.
So, it is usually best to first test your air using either a Short Term or Long Term test. If the results are elevated and you have a well water source, you should consider testing your water for radon.