Sea ice that has been above sea level for some time, like this heap at Shackleton's Ocean Camp, tends to lose most of its salt and can be melted for drinking water. This is essential, because despite holding some 70 percent of all the world's freshwater, Antarctica gets about as much precipitation as your average desert, and on sea ice all that freshwater locked up in snow is not available anyway. In summer, the melting of old sea ice can form freshwater pools on the surface of newer ice, but in winter you can't rely on these or even on significant snowfall. And never eat snow; you'll expend more energy than you can afford just melting it in your mouth.
The lack of freshwater increases the possibility that you'll become dehydrated. Your chances are already high, because the Antarctic's low humidity, high winds, and acute sunlight work in conjunction with your body's lowered thirst drive and hormonal changes to increase your susceptibility to the condition. According to Survival in Antarctica, "a 2.5 percent loss of body fluid by weight results in a 25 percent loss of abilities." Dehydration can make you feel tired and less sharp mentally, and it can reduce your tolerance to cold, thereby increasing your chances of contracting frostbite or hypothermia.