The strait between Shikoku and Awaji Island produces some of the largest tidal whirlpools on the planet — vortices that during spring tides can stretch 20 metres in diameter and roar audibly from a kilometre away. They are at their most dramatic for an hour either side of high or low tide; check the day's schedule before you go because outside that window there is little to see.
You can view the whirlpools from above, walking out along the glass-floored Uzu-no-Michi promenade slung beneath the Naruto Bridge, or from below on one of the sightseeing boats that motor up to the edge of the swirl. From central Tokushima it is about 40 minutes by bus to Naruto Park.