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Why is it so easy to float in the Dead Sea?
Salinity is around ten times that of the ocean, which makes the water dense.
Buoyancy depends on density. A person floats when the liquid around them is denser than their body, and the Dead Sea is far denser than ordinary seawater because it contains an enormous amount of dissolved salt. NOAA gives its surface water as several times saltier than the ocean, and deeper layers near saturation, with salinity of roughly 300 parts per thousand or more, compared with about 35 for the open ocean.
The 'ten times' figure in the short answer is a round number: sources vary, and values such as 'about nine times' or '5 to 9 times' depend on the layer and the year, and the lake's salinity has changed as its level has dropped. The cause of the high salinity is that the lake is a terminal basin. Water from the Jordan River and springs enters, but none leaves except by evaporation, which leaves the minerals behind.
Floating is easy but not entirely comfortable. Swimmers tend to ride high, and it is difficult to stay upright or swim normally, while the water stings cuts and eyes. The lake's surface sits roughly 430 meters (about 1,400 feet) below sea level, and the level has been falling by around a meter per year in recent decades. Despite its name, some microbes and salt-tolerant organisms live there.