This paper presents an economic assessment of seawater thermal energy storage (TES) integrated with industrial heat pumps to couple renewable electricity generation with urban district heating networks. Using Amsterdam as a case study, we develop a techno-economic model leveraging real-world data on electricity prices, heat demand, and system costs. Our findings show that large-scale TES using seawater as a storage medium significantly enhances district heating economics through energy arbitrage and operational flexibility. The optimal configuration yields a net present value (NPV) of EUR 466 million over 30 years and a payback period under 6 years. Thermal storage increases NPV by 17% compared to systems without storage, while within-day load shifting further boosts economic value by 23%. Accurate demand and price forecasting is critical, as forecasting errors can reduce NPV by 13.7%. The proposed system is scalable and well suited for coastal cities, offering a sustainable, space-efficient solution for urban decarbonization and addressing renewable energy overproduction.