The structure is intended to be used all year round on Seattle's lakes and can accommodate up to six people. It is called WA Sauna, a name derived from the abbreviation for Washington State, where Seattle is located.
It follows the growing trend among architects to explore the possibilities afforded by building on water rather than land.
"Following in the Scandinavian tradition of saunas as a place for gathering, WA Sauna provides a place for Seattle's community to share a unique experience on the water," said goCstudio, a firm founded in 2012 by Jon Gentry and Aimée O'Carroll.
The floating vessel totals 240 square feet (22 square metres) and is 14 feet high (four metres). It weighs approximately 4,500 pounds (2,040 kilograms).
Inspired by the concepts of fire, water and community, the designers aimed to build a structure that engaged the local waterways and encouraged people to use them throughout the year.
"The idea for the floating sauna was born on a cold and wet winter's day in January 2014," added the firm. "Combining our love of the water, the relaxing dry heat of saunas, and floating structures, the project began to take shape."
The $25,000 (£17,000) project was funded through community donations and a Kickstarter campaign hosted in the fall of 2014.
The deck consists of a pre-manufactured aluminium frame and marine-grade plywood with a clear varnish. Boats and kayaks can be tied up to the deck.
The floating structure is powered by a 36-volt electric trolling motor. More than two dozen 208-litre plastic drums keep the vessel afloat.
The siding is made of treated plywood with a semi-transparent ebony stain, and the windows contain clear acrylic panels.
Spruce was used to clad the interior and to form the benches. A wood-burning stove heats the space.
Users can easily exit the vessel via a door or side hatch and dive into the cool water.
"The lake serves as a natural cold plunge after heating up in the sauna," said the designers.
The sauna is typically moored at Lake Union in Seattle. The designers generally take the vessel on two- to three-hour trips around the 580-acre freshwater lake.
The structure was built by studio employees and skilled volunteers. It was erected within a warehouse owned by the local brewery, Hilliard's, which allowed the team to use the space for free.
One of the greatest challenges was getting the structure to the lakefront for the first time.
"Maneuvering a 14-foot high, 4,500-pound structure from the warehouse to the public boat ramp and into the water was a challenging process," said the studio.
"Towed on six steel casters with a 1980 Volkswagen Vanagon, we slowly crept along at dawn making the eight-block trip to the boat ramp in just under three hours."
Feeling the structure gently floating on the water was the "most exciting moment of the build process," the firm added.
Rising sea levels and a shortage of development sites are leading to a surge of interest in floating buildings, with proposals ranging from mass housing on London's canals to entire amphibious cities in China.
Photography is by Kevin Scott.
Team: Jon Gentry and Aimée O'Carroll
Builder: goCstudio, Greg Lewis
Engineer: Kevin Winner, Swenson Say Faget