Dear student,
We can easily float in a dead sea because dead sea has a salty water and thus the water of dead sea is much denser than that of normal water which makes people easy to float into it. And according to Archimedes Principle, the buoyant force on an object in a fluid is equal to the weight of the fluid that an object displaces. So, the more dense and amount of fluid one displaces the more the buoyant force will act on it which enables us to float easily.
Regards