Via Ars Technica
"The key issue in all of this is the density of water, which drives the cycling of the oceans. Given similar temperatures, salt water is more dense; given similar salt content, cold water will be more dense. These density differences are large enough that if you have a patch of warm, fresh water sitting on top of cold, salty water, the two bodies won't mix very efficiently. The water ends up layered, or stratified."