The stratification of upper layer of coastal water can be regarded as an unused energy source for heat pump systems near coastal area in the summer. This study examines feasibility of heat pump system for coastal area using a cold water mass beneath the thermal stratified upper water. The field observations for thermalcharacteristics in the harbor were conduced through the summer season. The characteristics of stratified water are investigated by the field observations and numerical simulations. The breakdown of stratification is also examined by the numerical simulations for the heat pump system using the coastal water.