A portable floating hot tub that is easy to assemble and take down and can be used in environments with extremely cold and turbulent water. The hot tub has three main components: a collapsible frame made of connected rigid members, an insulated liner sized and shaped to closely fit against the deployed frame and form a subassembly, and an inflatable pontoon that may have a central aperture for receiving the subassembly. The pontoon is first inflated and placed on the body of water, after which the frame and liner subassembly are placed over the central aperture. Filling the internal volume defined by the insulated liner with water causes the subassembly to descend within the pontoon. Cooperating means on the subassembly and/or pontoon prevent the subassembly from passing completely through the central aperture. A source of heated water such as a portable heater having a heating capacity of at least 30,000 BTUs rapidly heats up the water within the hot tub. The system components can be transported in one or more carry bags of less than 100 pounds and deployed and taken down by one person in an extremely small space, such as on a boat.