Yep, you can get hypothermia in temps much warmer than you'd think. But even so I wouldn't have blinked if they'd said "50 degrees Fahrenheit", since that's a pretty standard -- okay, the only word that fits there is metric, heh. I remember being told something similar as a kid.

But 50 C - yikes!

the rule of thumb that without protective gear you can stay only as many minutes as the water is in Celsius, or you'll get too cold, even if the water is at summer temperatures.

Oh, I am totally using that, thanks! Much better than "if your lips start turning blue, get out of the water". (Although if your lips start turning blue, do get out of the water...)