That all homeowners should test their water for radon is one of many radon myths abounding out there. In fact, most homeowners only need to test their water for radon after they have done an air radon test. The air test is easier to conduct and may be the only one most homeowners need to have done.
Unless you live on well water, you likely receive your water supply from the local municipality, which tests the water. They send out an annual report indicating all of the items they test in the water to keep you and your family safe. Waterborne radon is not as dangerous as radon in the air making it more important to have an air test done rather than a water test.