The Naruto whirlpools occur along the Shikoku coast of the Naruto Strait, and are created by the large volumes of water moving between the Seto Inland Sea and the Pacific Ocean between high and low tide, combined with the unique underwater geography of the narrow strait.
According to the change of tides, the whirlpools occur roughly every six hours and can typically be seen once in the morning and once in the afternoon for an hour or two. The whirlpools vary in size, depending on the intensity of the tides. They tend to be larger in summer than in winter, and are largest during spring tides, which occur every two weeks.
Under ideal conditions, whirlpools of up to 20 meters in diameters can be observed. On the other hand, there is not much to see on calm days or outside of the peak times. Therefore, it is of great importance to check the whirlpool schedules (see links below) before making a visit to Naruto.
The whirlpools are not dangerous to boats and are best viewed from sightseeing cruises which get right up next to and among the whirlpools. There are two companies that run tours from nearby piers on Shikoku and one that operates from Awaji Island on the opposite side of the strait.
On the Shikoku side, Uzushio Kisen operates small boats with water level decks from a pier just outside Naruto Park. A little bit further away lies the pier of Uzushio Kankosen, which operates the "Aqua Eddy" (medium sized boats with two decks and underwater windows) and "Wonder Naruto" (large sized boats with two decks) to the whirlpools.
As the whirlpools are best viewed from a higher vantage point, it is recommended to take a "Wonder Naruto" boat tour which offers the best views from its upper first-class deck at an additional charge. Boat tours by both companies last 20 to 30 minutes with two departures per hour.
The next best views are from Uzu no Michi, an enclosed walkway that extends under the Onaruto Bridge and overlooks the whirlpools. The walkway's observation room sits 45 meters above the whirlpools below which can be seen through glass windows set in the floor.
Naruto Park (Naruto Koen), the park encompassing the attractions around the whirlpools, also has a number of walking trails and viewpoints found around it with views of the whirlpools and Onaruto Bridge. The best views of the swirling water are from the Senjojiki Observatory. Another viewpoint, Eska Hill, is accessible via a 68 meter long, paid escalator, but its restaurants and shops appear rather run down.
Also found in the park is the Onaruto Bridge Museum Eddy, which is devoted to the construction of the Onaruto Bridge, the strait and its whirlpools. Last but not least, the Otsuka Museum of Art, Japan's largest museum, stands out toward the park's southern edge and displays nothing but reproductions of Western Art.