<p>With an area of about 77,000 km<sup>2</sup>, the Bohai Sea includes three bays: Laizhou Bay, Bohai Bay and Liaodong Bay. In this study, <sup>228</sup>Ra, &#948;D, &#948;<sup>18</sup>O and salinity data were collected from surface seawater in the entire Bohai Sea, river water, and groundwater along its coastline in August 2017. Based on the spatial distributions of &#948;D, &#948;<sup>18</sup>O, and salinity in surface water in the entire Bohai Sea and &#948;D-salinity relations, the marine hydrological processes were investigated and the members of river water and groundwater for &#948;D and &#948;<sup>18</sup>O were determined. The steady-state mass-balance models of &#948;D, &#948;<sup>18</sup>O and salinity are given and used to estimate submarine fresh groundwater discharges and the flushing times of the entire Bohai Sea and its three bays. Based on the results of the flushing times, the steady-state mass-balance model of <sup>228</sup>Ra is used to estimate submarine groundwater discharges of the entire Bohai Sea and its three bays.</p>