Francesco A. Pia, Ph.D., has formulated The Instinctive Drowning Response. It describes what people do to avoid drowning and is not like what most of us believe drowning looks like. Apparently, in 10 percent of drownings people nearby have no idea the victim is in trouble.
Read about the Instinctive Drowning Response in this article by Slate: Drowning Doesn’t Look Like Drowning.
What does drowning look like?
How to prevent drowning
Note: this update to an earlier post includes videos from Slate.