Rips. They're one of the first things new swimmers are warned about, right behind staying between the flags. That said, you might not comprehend just how dangerous a rip can be, fortunately, we have photos like the one above to highlight exactly what the fuss is about.
(Click on the image for a larger version.)
The photo, snapped by paraglider Scott Patman, was posted on the "Dr Rip's Science of the Surf" Facebook page, run by UNSW surf scientist Dr Rob Brander. What it shows are rips pulling sand from the coast out to sea, forming a giant swirl of sediment. According to Dr Brander, the image was taken south of Perth from about 300m in the air, with the swirl positioned 150-200m from the shore.
I've had a long-life fear of the ocean and, well, seeing this hasn't helped matters. So yeah, I'll be sticking to the shallow end of the kid's pool for now.