Objects float better in salt water than in fresh water because salt adds mass to water and makes it denser. This density causes objects to float better on the surface. The denser the water, the easier it is for objects to float on top.
Less salt in water means things are not as buoyant, while more salt dissolved in water elevates objects. Density is defined as mass spread out over a particular volume. More particles within a smaller area leads to denser objects. Less dense objects float on more dense objects.
Simple science experiments show how salt causes things to float better. Take six containers of equal volume and add enough water to float an egg. Add increasing amounts of salt to five containers while leaving the sixth with no salt. Measure how each egg floats below and above the water line.
The Dead Sea in Israel is an example of dense salt water. The density of minerals in the Dead Sea are the highest concentration in the world, according to the Jewish Virtual Library. The reason more minerals stay in the water of the Dead Sea is that more water is evaporated from the sea than what goes into it. This evaporation decreases the amount of water and increases density of dissolved minerals.