Salt water does not evaporate faster than fresh water; in fact, fresh water always evaporates faster than salt water. This is because of the difference between the salt and water molecules. Water is a slightly volatile substance, meaning that it is capable of evaporation, while salt is a nonvolatile substance, so it is not as capable of evaporation.
Evaporation takes place on the surface of a substance. Fresh water has only water molecules on its surface area, which lets it evaporate easily. Salt water, on the other hand, has both salt and water molecules on its surface area. The salt molecules take up part of the surface area and prevent the water molecules from evaporating as quickly, which is why salt water always evaporates more slowly than fresh water.