The official name for this spot is The Floating Torii Gate of Oouo Shrine.
Not sure what the story is with these, every time I read up on them the story seems slightly different and it never makes any sense. Something about blessing the waters for better fishing, or there is something about some giant fish. That’s usually how it goes with historical sites in Japan. A giant catfish, a giant fish, a mountain god, etc. Fun stuff.