A whirlpool is a body of swirling water produced by the meeting of opposing currents. The vast majority of whirlpools are not very powerful and very small whirlpools can easily be seen 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 any whirlpool that has a downdraft.
In oceans, in narrow straits, with fast flowing water, whirlpools are normally caused by tides; there are few stories of large ships ever being sucked into such a maelstrom, although smaller craft are in danger. Smaller whirlpools also appear at the base of many waterfalls and can also be observed downstream from manmade structures such as weirs and dams. In the case of powerful waterfalls, like Niagara Falls, these whirlpools can be quite strong.