Here’s a hot (or at least comfortably warm) attraction for Seattle’s cold winter months: a soak in 104-degree water on Lake Union in the city’s original hot tub boat.

Hot Tub Boats is the brainchild of Adam Karpenske, who — after moving back to Seattle after working on super yachts in the Caribbean for more than a year — started “tossing around” the idea of building a floating hot tub to escape a dreary Seattle winter. According to Mount Baker Experience Magazine, Karpenske created the world’s first hot tub boat here in Seattle. Its first excursion launched on New Year’s Eve 2011.

Two months later, Karpenske created a website, though he wasn’t sure he had a sustainable business. He was pleasantly surprised when the site began garnering 30,000 views a day.

“You go from barely anybody talking about it to what felt like the whole world talking about it and non-stop emails coming in,” Karpenske recalls. “We had so much traffic on our website and that much response to a product that we were still trying to figure out. It was crazy.”

Today, Hot Tub Boats runs eight to 10 boats off Lake Union. It recently launched in Alameda, Calif., just outside Oakland. The fiberglass boats are available for parties of six or fewer for two-hour cruises, and participants actually drive the boat themselves using a joystick. Kids are allowed. Alcohol is not, though food and beverages are. The company is upfront about one rule: no nudity.

“Just be good people out there,” Karpenske says with a laugh.