Why We Can't Swim and Drown In Cold Water?

It’s called the cold shock response.  When the cold receptors in your skin are all suddenly stimulated they cause an involuntary gasp and, for about a minute after that, hyperventilation. If you fall into chilly water, the cold shock response will kill you long before hypothermia does. Either that first gasping breath will fill your lungs with water (drowning you instantly), or the hyperventilation will make swimming almost impossible.  

source: BBC Knowledge Magazine