You've probably seen someone drowning in movies, but that's not what drowning really looks like. The Art of Manliness has put together a great infographic to help you learn to spot actual victims.
The overall takeaway is that drowning victims can't actually flail around or make a bunch of noise when drowning. Instead, watch for people with upright, still bodies (if you can see them) and heads that are very low to the surface of the water.
What Drowning REALLY Looks Like [Art of Manliness]