Sea salt is evaporated seawater, and unlike refined table salt it keeps the trace minerals — magnesium, calcium, potassium — that were dissolved alongside the sodium. The coarse grain is the point: it is abrasive enough to lift dead skin, which table salt, being fine, is not.
Three properties make it useful on feet. It exfoliates, taking off the dull built-up layer that makes heels look grey. It draws fluid out of swollen tissue by osmosis. And salt water is mildly antiseptic, which is why it has been used to clean minor wounds for as long as people have lived near the sea.
Epsom salt and sea salt are often used interchangeably, but they are not the same. Epsom is magnesium sulphate and better for aches; sea salt is sodium chloride and better for scrubbing.
