This study addresses the issue of energy waste caused by the direct discharge of seepage seawater from the drainage-type underwater tunnel constructed by the mining method, without effective utilization. The tunnel seepage seawater is utilized as a renewable heat and cold source, and a seawater-source heat pump system is proposed to provide heating (cooling) to adjacent buildings. The performance of the heat pump system was continuously monitored during the heating season, and the actual operational efficiency of the system was analyzed. The measured data shows that under winter heating conditions, the average room temperature in a typical month was 22.4 °C, fully meeting the design requirements. The energy consumption of the distribution system accounted for only 19% of the total system energy consumption, demonstrating excellent energy efficiency characteristics. Notably, the coefficient of performance (COP) of the heat pump unit in a typical month reached 3.6, fully verifying the feasibility and high efficiency of this technological solution in actual engineering applications.