The dead sea is also called the Salt Sea, because it is one of the world's saltiest bodies of water, with 33.7% salinity. Because of this, animals are unable to live in the dead sea, hence its name.
So why do you float on the dead sea?
Well, since there is so much salt in the dead sea, the water there is very dense compared to fresh water or ordinary seawater. Also, less objects or surface floats in the water. That means that we're less dense, so we can float on the dead sea.
Sources:
http://en.wikipedia.org/wiki/Dead_Sea
http://wiki.answers.com/Q/Why_do_you_float_on_the_Dead_Sea
http://en.wikipedia.org/wiki/File:Dead_sea_newspaper.jpg