This dataset provides high-resolution porewater salinity measurements used to investigate spatial variability within subterranean estuaries (STEs) across contrasting coastal settings. Field surveys were conducted at Marine Drive beach and Siali beach along the Odisha coast, India, and at Spiekeroog Beach, Germany. Sampling campaigns were performed during the post-monsoon season (October 2024) at Marine Drive and Siali beaches, during the pre-monsoon season (March 2026) at Marine Drive beach, and in June 2025 at Spiekeroog beach. Porewater was collected from the intertidal zone at 0.5 m sediment depth using a push-point sampler following meter-scale grid-based surveys to characterize lateral variations in fresh-saline mixing. Sampling locations were mapped using QField for QGIS connected via Bluetooth to a Trimble DA2 GNSS receiver with an active Catalyst subscription, enabling high-precision positioning and simultaneous recording of spatial coordinates with field measurements. Porewater salinity, electrical conductivity, and temperature were measured in situ using a WTW multiparameter probe (Xylem Analytics Germany). The measurements capture localized freshening zones and heterogeneous mixing patterns within STEs, highlighting the spatial complexity of fresh-saline mixing. This dataset supports the evaluation of lateral variability in SGD pathways and contributes to improved understanding of the spatial heterogeneity of STEs at the beach scale.