That fresh water does indeed float on salt water is used in two situations that I know of from personal experience and I learned at school that some European sailors who'd run out of water off (but out of sight of) the eastern coast of Sth America in the 16th or 17th century asked some fishermen for spare fresh water; they were told to drop a bucket over the side and found it was fresh; they were in the outflow of the Amazon.

The two situations? Coral atolls in the Pacific may be only a few metres above sea level and relatively permeable but contain a lens of fresh water (from rain) that's 'floating above' the salt water. When police divers in Melbourne wish to search the bottom of the Yarra below Dights Falls (the upper limit of tidal influence) they prefer to do so when the tide is in. Unkind people from outside Victoria assert the Yarra is the only river that flows upside down (the mud's in the top layers rather than the bottom; they also say "It's too thick to drink and too thin to plough!) because of its mud content.   The mud makes visibility almost nil but the tidal salt water flows underneath it and forces it up from the bottom; the saltwater has no mud and provides excellent visibility on the river bed.

Cheers, Rowan