Ships sailing over a layer of warm that sits on top of saltier, or colder, layers are slowed by waves formed at the boundary of the layers which form a gulf beneath the ship slowing them by anything up to 80%.
Fluid dynamicist Leo Maas at the Royal Netherlands Institute for Sea Research, and colleagues found that the same conditions also affect swimmers "We've been considering the possibility that the drownings of strong swimmers in fair-weather conditions might be the result of dead water, but until now, we weren't really sure if this phenomenon could strike something as small as a single human," says Maas. "Now it seems that it can."
I suppose if a ship can be slowed by the phenomenon then why not people? What is sure, the article is the perfect lead in to what must be the creepiest public information film ever made –a film made perfect by a Donald Pleasance voice over