Rechargeable seawater batteries (SWBs) offer a highly sustainable and intrinsically safe energy storage paradigm that leverages naturally abundant seawater, effectively bypassing critical raw material dependencies. In these systems, a redox-active sodium-biphenyl (Na-BP) anolyte is employed within the anode compartment isolated by a NASICON solid electrolyte to ensure high Na metal reversibility. However, under demanding operating conditions, SWBs remain susceptible to severe dendritic metal growth and subsequent NASICON solid electrolyte degradation. In this study, we have comprehensively investigated these failure mechanisms using X-ray computed tomography and electrochemical current distribution modeling. The results reveal that the high electronic conductivity of the Na-BP anolyte relative to its ionic conductivity induces a substantial ohmic (IR) drop disparity. This disparity effectively supersedes the interfacial kinetic barriers, driving the preferential Na deposition directly onto the NASICON surface. Subsequent localized metal aggregation generates significant mechanical stress, triggering catastrophic transgranular spallation and eventual short-circuiting of the NASICON solid electrolyte. Our findings highlight the mechanistic synergy between SWBs and all-solid-state batteries, and provide crucial insights for the advancement of both seawater-based and next-generation metal-based energy storage technologies.