from http://oceanservice.noaa.gov/facts/oceanfreeze.html and from

At least 15% of the ocean is covered by sea ice some part of the year.

While their ship was imprisoned in the ice off Antarctica, the crew of the Endurance melted the sea ice and used it for their drinking water. How is that possible? Why wasn't it too salty?

The reason has to do with the behavior of ocean water when it freezes. When salt water freezes, much of the salt is expelled from the ice or gets trapped in pockets of salty liquid water within the ice. Eventually, most of the salt makes its way into the water just under the ice. This creates a layer of water under the ice that is saltier than the water below it. Because this layer of cold salty water is very dense, it sinks to the bottom levels of the ocean. As it sinks, it is replaced by water that is either warmer, less salty, or both. As ice continues to form, expelling more salt into the surface waters, this new layer of water in turn becomes saltier and it begins to sink, too. In this way, water circulates under the sea ice in a cycle that lasts the entire Antarctic winter.