In the TV series Supernatural, the Winchester brothers regularly use salt in various forms to repel or kill evil spirits. Does this mean that a person who is at sea (and therefore surrounded by a high concentration of salt in the water and air) or at a salt lake will be safe from all except the most immune spirits and demons?
Firstly, they never kill a ghost with just salt, they are need to spread salt on the remains (bones, body or something associated with the ghost) of the ghost, then burn the remains.
On repeated occasions, someone asks the Winchesters 'Why use salt or iron bars to repel ghosts?' They answered, 'Because they're pure materials'. So I think diluted salt would not work like pure salt.
In later episodes you can see, in order for a ghost to move an object they need a to concentrate and/or be enraged; when something pure "touches" the ghost (iron, salt etc.), they lose the ability to move objects (or even appear corporeal) for a small period of time.
By the end of the season seven you can see all these things.
I seem to recall an episode about a ghost ship...
I guess it's possible that the ghost ship was sailing in fresh water. However, the cast iron rock salt coated panic room Bobby built seems pretty ghost/demon proof. Even that was protected with devils snares and other mysterious symbols, though.