An easy-to-handle experimental setup for the on-site determination of the radon concentration in water samples has been developed and tested. In contrast to laboratory techniques for radon-in-water determination such as liquid scintillation counting or gamma-ray spectroscopy, the described equipment and method allow for a fast and uncomplicated determination of the radon concentration in water samples immediately on site with satisfactory accuracy. The method is based on the water/air partitioning of radon, which is known quantitatively over a wide range of temperatures. With the aim to evaluate the reproducibility of the data and the time efficiency of the method an appropriate number of laboratory experiments have been carried out. The results are discussed in the paper. Furthermore, an on-site experiment was carried out in order to evaluate the field applicability of the developed experimental setup.