Watch sea whirlpools from a boat or a bridge
Where the tide rushes through the narrow Naruto Strait between Awaji Island and Shikoku, the water spins into whirlpools. Watch them from a sightseeing boat, or look straight down at them through a glass floor under the bridge. They are strongest for about an hour and a half either side of the fastest tide, and biggest in spring and autumn, so check the tide table; Naruto is about 2 hours from Osaka by highway bus.
A walkway runs 450 m inside the girders beneath the Onaruto Bridge's road deck, out over the strait. At the end, glass panels set in the floor let you look straight down at the whirlpools far below. The ticket window takes cash only.
A large sightseeing boat that sails under the Onaruto Bridge to the edge of the whirlpools, about 30 minutes there and back. No booking is needed, and the operator's English website shows each day's tide times so you can pick a sailing.
A small, fast boat from Kameura fishing harbor that rides low on the water, so you feel the speed of the current close up. When a boat is full you wait for the next one, and it takes cash only.
A lookout on a hill in Naruto Park that takes in the Onaruto Bridge and the whole strait at once. Stop here before or after a boat ride or the walkway to see how the narrow channel squeezes the tide into whirlpools.