In this paper, for the first time, we investigate the inherent ionic conductivity of seawater-based cementitious composites containing marine sand aggregates, when air cured over 28 days with the objective of uncovering new functionalities. Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and density functional theory (DFT) were employed to elucidate the ionic conduction mechanisms in this material system and characterize its stress self-sensing and electrochemical charge retention capabilities. The results revealed that the marine aggregates are not ionic conductive materials; however, they facilitate improved electrochemical response through enhanced formation of highly ion-exchanging calcium-silicate-hydrate (C-S-H) phases, coupled with integrated porous channels that enable sustained ion mobility despite drying. This synergistic ion transport yielded a bulk ionic resistivity around 25 kΩ⊡cm at room temperature, which lies in typical ranges seen in solid-state electrolytes for battery systems. Controlled compressive loading indicates appreciable self-sensing capacity at low-stress levels, suggesting applicability to detect the onset of mechanical damage. Negligible charge leakage upon 28 days of curing further demonstrates the electrical energy storage potential of the sea-based cement. By harnessing locally available seawater and marine sand resources to develop ionic conductive cementitious composites, this work provides the framework to optimize durable multifunctionality for sensing and electrical energy storage in reinforced concrete infrastructure. This in return improves the sustainability and energy efficiency of the built environment.