|Naruto Strait whirlpool from Mantokun, Wikimedia|
The natural whirlpools in Japan occur in the Naruto Strait, a channel between two smal islands, Tokushima and Awiji. The straight is less than 1 mile in width, and connects the Pacific Ocean and the Inland Sea, a body of water that separates Honshu and Shikoku. Tides here have a range up to 5.6 feet, creating a difference in water level of nearly 5' between the Inland Sea and the Pacific. Because the strait is so narrow, water rushes through it at 8-9 mph, four times per day, and during spring tides the speed can reach 12 mph. At that time, whirlpools up to 66' in diameter can occur.
The whirlpool in your bathtub is not a good analog for most natural whirlpools because it is caused by the exodus of water down your drain. However, there was one similar whirlpool in a lake in Louisiana in 1980 when a drilling rig broke through the roof of an underground salt mine. The lake drained, just like bathwater, down through the salt mine and a whirlpool developed. Although some boats and semi trailers were caught up in it, no lives were lost.