A spa or hot tub system includes a tub forming a cavity that holds a fluid such as water therein, and a water circulation system provides for circulation of water within the tub. The hot tub includes inner and outer surfaces, and defines an internal space. A heat pump system is operably connected to the water circulation system, and heats the water in the hot tub. The water side heat exchangers of the heat pump may include a polymer housing, and coiled inner elements for refrigerant. The coils may be made of a corrosion-resistant metal tubing or the like. The system may include a conventional electric heating element in addition to the heat pump to provide additional heating capacity.