Seawater is salty because rainwater dissolves the salt minerals found in soil and rocks as it travels through rivers and streams en route to the sea. This method of mineral dissolution is called weathering.
Only a small percentage of the sea’s salt is added annually through weathering. According to the University of Texas at Dallas, it took approximately 200 to 300 years to create all of the salt that is currently located in the world's seas.
The Great Salt Lake and the Dead Sea are about 10 times saltier than other seas and areas of saltwater because they have no outlets. The only water that escapes is through evaporation.