A whirlpool is a body of rotating water produced by opposing currents or a current running into an obstacle. Small whirlpools form when a bath or a sink is draining. More powerful ones in seas or oceans may be termed 'maelstroms'. 'Vortex' is the proper term for a whirlpool that has a downdraft.

In narrow ocean straits with fast flowing water, whirlpools are often caused by tides. Many stories tell of ships being sucked into a maelstrom, although only smaller craft are actually in danger. Smaller whirlpools appear at river rapids and can be observed downstream of artificial structures such as weirs and dams. Large cataracts, such as Niagara Falls, produce strong whirlpools.

The Maelstrom of Saltstraumen is Earth's strongest maelstrom. It is located close to the Arctic Circle, 33 km (20 mi) round the bay on Highway 17, south-east of the city of Bodø, Norway. The strait at its narrowest is 150 m (490 ft) in width and water "funnels" through the channel four times a day. It is estimated that 400 million cubic metres (110 billion US gallons) of water passes the narrow strait during this event.

More Info: en.wikipedia.org