Subsurface dam is a technically feasible and widely used solution to prevent seawater intrusion (SWI) in coastal areas. The effectiveness of subsurface dams and the related environmental problems have been individually established, although static oceanic conditions are usually considered. In this study, sea boundary conditions, including static and tidal scenarios, were compared to comprehensively assess subsurface dams for SWI control and upstream groundwater pollution, thus extending previous findings. The index of fresh groundwater discharge was used as a proxy to assess the environmental problems associated with subsurface dams. Laboratory experiments and numerical simulations were performed to analyze the effects of dam height, location, and dispersivity. The results revealed that the effectiveness of subsurface dams could be described through a so called effective and increased discharge height range (E&IDHR), which was depending on dam location. Showing properties within the E&IDHR, subsurface dams could not only effectively prevent the invasive saltwater, but also increase the amount of fresh groundwater discharge relative to the scenario prior to dam construction. Tidal fluctuation significantly altered the E&IDHR, and the lower and upper thresholds of the E&IDHR were lower than those of the no-tide cases. The lower threshold was the minimum effective height, which was close to the saltwater wedge (SW) height determined by 95% isohaline (95% SW height) for the no-tide cases and the 20% SW height for the tide cases. This difference was attributed to intensive salt-freshwater mixing driven by tides. The upper threshold of the E&IDHR was usually close to 116% of the 5% SW height. Acknowledging the E&IDHR, applied studies may benefit from numerical and parametric uncertainties when designing subsurface dams, while still attaining optimal effectiveness to prevent SWI and upstream groundwater pollution.