Coastal ecohydrology in arid environments, where riverine inputs are small, are shaped by complex interplays between groundwater, surface water, and tidal dynamics. Intertidal sediments typically act as sponges, regulating water exchange through sheet flow and percolation. In our study in Exmouth, WA we monitored water level and salinity in a series of shallow wells and measured mangrove sap flow over several years. Our data revealed a strong hydrological disconnection between groundwater and surface ocean water. Instead of sheet flow contributing to groundwater recharge, lunar tidal cycles control hyper-saline groundwater pumping to the surface during big tides, creating temporary connectivity. This process has major implications for coastal flooding and wetland dynamics, as the lack of direct infiltration means oceanic floodwaters persist longer, prolonging inundation and potentially shifting wetland structure and function due to evapotranspiration driven salinisation of the groundwater. As sea levels rise, more frequent and intense tidal-driven groundwater emergence could exacerbate coastal flooding, accelerate wetland degradation, and reduce carbon sequestration capacity by altering vegetation function, highlighting the need for new coastal flood management strategies that account for tidal-driven groundwater fluxes, as traditional drainage and recharge-based solutions may be ineffective in hydrologically disconnected coastal systems facing increasing flood risk.