The Mediterranean Sea is very warm so evaporation occurs more rapidly than in most other bodies of water, therefore more salt is left behind. Because it is mostly enclosed by land this warm, dense, salty water stays there for a long while, up to 100 years before returning to the Atlantic Ocean. It is gradually replaced by less salty and dense Atlantic water in the Strait of Gibraltar.
Because it is so salty you float more in it and it is lovely to swim in. Salt is an electrolyte and so the saltier water is the more conductive it is. Swimming in the sea strongly connects us into the Earth’s electricity and swimming on the Mediterranean exceptionally so. A very healing relaxing experience.