… I did not need a new plot bunny…
How about…static salt crystals are an impenetrable, uncrossable obstacle, but salt in motion is not.
Salt crystals are the obstacles, whereas salt in solution is not a problem. This might have something to do with the actual shape/structure of the crystals themselves, once they reach a certain size or number of crystaline iterations within a single crystal. So maybe…sodium chloride’s crystaline structure holds supernatural power, but sodium flouride’s doesn’t? Which would explain why demons don’t react to standard drinking water in the US (unless it’s been blessed, of course.) Also, the ocean has a mixture of salts containing calcium, magnesium and potassium, so maybe the mixture would alter the structure of the crystals enough or keep the concentrations of sodium chloride in solution low enough comparatively, that the supernatural power held by the table salt crystals would be overwhelmed?
I just figure, if you’ve gotta put lines in front of your windows and doors, grab the table salt, ‘cause the epsom salt and the baking soda are suspect.
The reason (and by “reason” I mean “real-world lore that SPN presumably borrowed from) salt repels evil spirits is because it’s pure, and purity repels evil. So salt in solution, especially a solution as complex and varied as an oceean, probably would not harm a demon.
Then again, if you go with that lore, table salt wouldn’t either because it’s got iodine in it and is thus no longer pure. Gotta stick with kosher salt all the way.
OMG, as a biology teacher, this whole post gives me shivers