Undertow (water waves)
The word undertow is used in at least two senses, both of them in the context of shallow ocean water near beaches. Firstly, "undertow" is an old-fashioned term that is no longer used in physical oceanography, a term for what is now known as a rip current, i.e. a narrow flow of fast-moving water that can pull a swimmer rapidly out to sea.
Secondly, "undertow" is a word used by many people to refer to what they think are extremely common but dangerous shallow-water near-beach conditions. This misconception or myth holds that where there are waves there is also a water flow or current that can prove fatal by pulling a person down vertically and holding that person underwater until they drown. People therefore believe that "undertow" is a very real danger which is responsible for many drownings.
On the United States Lifesaving Association website (emphasis added) it is explained that "undertow" is a myth:
"A rip current is a horizontal current. Rip currents do not pull people under the water–-they pull people away from shore. Drowning deaths occur when people pulled offshore are unable to keep themselves afloat and swim to shore. This may be due to any combination of fear, panic, exhaustion, or lack of swimming skills."
"In some regions rip currents are referred to by other, incorrect terms such as rip tides and undertow. We encourage exclusive use of the correct term – rip currents. Use of other terms may confuse people and negatively impact public education efforts."
Under the wrong circumstances, a person can of course drown in any body of water, even a puddle, but the great majority of drownings close to beaches happen because someone gets caught in a rip current which starts to carry them out to sea. If a person caught in a rip current does not understand that they can easily exit the rip by swimming at a right angle to the flow, they may panic and drown, or they may exhaust themselves by trying unsuccessfully to swim against it and then eventually give up and drown.
A rip tide or rip current is formed by water returning out to sea after it has been pushed ashore by breaking waves. If there is an area where water can flow back out to sea easily (such as a break in a sand bar) then a rip current will form, a fairly fast-moving but fairly narrow current of water that is running out to sea. 
The word "undertow" is also often used as a metaphor for hidden negative emotional currents.
- Goldthwait, J.W. (1908), "Present Shore Line by", Illinois State Geological Survey Bulletin (University of Illinois) 7: 32–33 Original from Harvard University
- http://www.usla.org/?page=RIPCURRENTS accessed 2 January 2014
- "United States Lifesaving Association's - Rip Currents". www.usla.org. Retrieved 8 July 2009.