Itsukushima Shrine, on an island off Hiroshima commonly known as Miyajima, was founded in 593 A.D. and is dedicated to a kami (god) who protects against sea disasters and wars.
"The priest explained that there are bigger Shinto shrines in Japan, but the unique quality of Itsukushima is its construction over the water. The reason for this, he said, is that traditionally the whole island was considered a shrine, to which Itsukushima is the point of entry." (Ferrar)