These ingenious structures are platforms that “float”, the surface of the floating deck about a foot above water. The platform is hitched to a pavilion comprised of four large beams anchored into the riverbed, and which support the roof. The illustration below is an overhead view, roof removed.
Wooden collars with rollers, attached to the platform at each of the four corners, surround the beam and allow the platform to rise and fall with the tides. There is some limited lateral motion as well, like a boat gently bumping against a dock, that is very relaxing, rather otherworldly really, at least in calm weather. I sought shelter there in a storm once, and it was pretty wild - waves washing over the deck, rain lashing in, the platform tossing and the pavilion shuddering like a ship at sea. They’re very sturdy, though, and have withstood everything, including hurricanes, for twenty years.
I pack a lunch and a friend; or else a hammock, a book, and an irresistible tendency to doze off... Waking up here is momentarily, and deliciously, disorienting.