With the rapid development of the global nuclear energy industry and the implementation of Japan's Fukushima nuclear wastewater discharge plan, the migration patterns and environmental impacts of the radioactive isotope tritium (³H) in coastal waters have garnered significant attention. Meanwhile, the quantitative characterization of terrestrial material fluxes through submarine groundwater discharge (SGD) represents an international frontier in marine science. As a core area for clustered nuclear power development in southeastern China, the region faces dual pressures from tritium released during the operation of nuclear facilities and tritium input from transboundary nuclear pollution. However, systematic studies on the distribution and transport characteristics of tritium in groundwater and surface water systems in this region remain scarce. This study focuses on typical areas including Hailing Bay (Guangdong Province), the Changjiang Estuary, and the coastal zones of Zhejiang and Fujian Provinces to comprehensively investigate the spatiotemporal distribution of ³H in groundwater. The study also evaluated groundwater ages in the eastern coastal areas, quantitatively estimated the contribution of SGD to tritium levels in coastal waters, and revealed its implications for regional ecological security and water resource management.