The marine differential electric dipole (DED) is applied for the first time to study a subseafloor groundwater body in the coastal region of Bat Yam, Israel. Previous marine long-offset transient electromagnetic applications detected this freshwater body underneath the Mediterranean seafloor. We have applied the novel DED method for the first time in the marine environment to further investigate this natural phenomenon. The main objectives are to locate the freshwater-seawater interface at the western aquifer edge and to identify the mechanism controlling this freshwater occurrence beneath the seafloor. The acquired step-on signals allow one to detect the freshwater body in the vicinity of the Israeli coastline at a depth of approximately 70 m beneath the seafloor. However, aquifer thickness is only poorly determined and may vary between 40 and 100 m. A lateral resistivity contrast is observable between adjacent 1D inversion models and also apparent in data profile curves that constrain the seaward extent of the detected resistive body to a distance of less than 4 km from the coastline. A subsequent 2.5D forward-modeling study aims to find a subseafloor resistivity distribution that adequately explains all measured DED data simultaneously. The results further constrain the lateral extent of the resistive aquifer to approximately 3.6–3.7 km from the Israeli coast. Furthermore, the data indicate that the aquifer system may be susceptible to seawater intrusion, as a superior data fit is achieved if a brackish water zone of approximately with a lateral extent of less than 300 m is located at the head of the freshwater body.