Whirlpool

A whirlpool is a body of rotating water. It is usually produced by ocean tides. A bigger whirlpool is called a maelstrom. It is also produced by a current running into an obstacle or opposing currents. 
Whirlpools as a threat.
Many stories tell of ships being sucked into a maelstrom, although only smaller craft are actually in danger. If you swim close to it, it will bring you underwater. You will drown and surface after a few seconds.