In quiet waters that don't mix very much, such as the Baltic Sea, fresh water from rivers tends to stay on top, floating on the salt water from the ocean. This is caused by the difference in their...
As anyone who has read about historical sailing ships knows, fresh water goes bad in about one week, depending on the container and environment. As every educated person knows, sailing ships always...
Overnight, tops. They start to absorb water and turn brown if you leave them longer. There is a catering product called Dry White my aunt used when she ran a restaurant.