So
Some lakes, such as the Great Salt Lake, accumulate soluble minerals such as salt. In
those lakes, people find it much easier to float than when they are in fresh water. Why is
this the case?

The reason for people to swim easier in salt water than fresh water is because of buoyancy

Explanation:

In fresh water there is lack of minerals and has fresh water alone. The density of fresh water is 1000 kg/m³.  Hence, in fresh water  cannot exert the suitable buoyancy for the swimmer to float easier than that of salt water.

But in Salt water due to enrichment of salts  and minerals it is found that salt water has more density than fresh water. Here the salt water offers more buoyancy to the swimmer to lift him up in the water surface and to swim faster and easier than fresh water.

It is similar to that egg floats in the salt water and sinks inside the fresh water because of its own body weight.