The Dead Sea is almost 10 times saltier than the ocean because of its lack of adequate drainage. It, and the other salty seas such as the Great Salt Lake and the Salton Sea, receives river water that is only slightly salty, but it lacks an outlet that would permit the water to escape its basin. This concentrates dissolved salts in an effect known as a sump.
The Dead Sea rests in the deepest continental basin on Earth. At 1,300 feet below sea level, the Dead Sea basin is able to accept flowing water from uphill sources, such as the Sea of Galilee. However, there is no downhill drainage area into which the sea can drain. This means that the only way for water to escape the Dead Sea basin is by evaporation.
When river water, which is always slightly salty, flows into a sump, all of its dissolved salts are carried with it. If the lake has an outlet, these salts can be carried along to the sea, and the lake remains fresh. Without an outlet, however, slightly brackish water can only evaporate, leaving behind its salts. This concentrates salt in the basin as more water flows in, only to deposit more salt and evaporate.