Airborne radon gas is more common than radon in water, which makes it more of a concern. When water containing radon comes into contact with indoor air, it will release most of the radon gas, which you're at risk of breathing in. However, the remaining radon in water is still dangerous, and can still have serious health effects when consumed.
If you're looking for a ballpark figure, any more than 300 to 10,000 pCi/L of radon in your water is considered potentially dangerous.