Set atop a steel catamaran hull, the timber structure has a distinctive bow-shaped roof that sweeps downwards to cover a central, glazed sauna room.
"The client, a creative person from the Stockholm tech scene, had a dream about creating a truly extraordinary floating sauna experience, something elegant and refined, yet unexpected," said Sandellsandberg architect Johan Strandlund.
Intended to blend in with the many surrounding forests, the exterior of the Big Branzino is clad in pine planks and opens onto two areas of decking through fully-glazed ends at the bow and stern.
Two smaller spaces containing a shower and bathroom sit on either side of the main sauna area, which contains two-tiered wooden seating around a bespoke stove.
A ladder at one end leads up to the roof, where there is a seating area that provides uninterrupted views of the landscape. This can be folded and easily stowed "when the occasion calls for dancing".
"The exterior is a rugged pine to blend in with the surrounding nature, [while] the interiors and furniture is made of western red cedar due to its great characteristics for sauna and outdoor use," said Sandellsandberg.
"The design revolves around its distinct shape. The lowest point of the arch creates warmth and closeness to the glow of the fire, and as the arch rises to the sides it allows the stars of the night sky to be admired from the inside," it continued.
A wheel and controls housed in a cedar box at the front of the floating sauna allow guests to drift around the archipelago with a maximum speed of around five miles per hour.
"It can travel freely at its captain's discretion, from a few hours of sauna with a view to adventurous, long trips," Standlund told Dezeen.
"When it was delivered from the shipyard to the client it was a two-day trip, [which] allowed us to discover the range of its experience. Other than the sauna, the fireplace provides warmth for overnight trips," he added.
Founded in 1995 by Thomas Sandell and Ulf Sandberg, Sandellsandberg is a Swedish architecture studio currently led by Louise Arnell, Kenny Kåhre, Anders Rotstein and Thomas Sandell.
Previous projects by the studio include a red-painted timber entrance pavilion to a hotel and nature reserve in Blekinge. Lausanne University of Art and Design graduate Trolle Rudebeck Haar built a pre-fabricated floating sauna in Switzerland that offers views over the alps.
The photography is by Filip Gränström.