So the salt in the water makes the water denser (aka thicker) then fresh water. In case you didn't know I'll tell you right now, it is easier for objects to float on denser liquids. Our bodies are actually slightly heavier then the less thick fresh water so that is why our body sinks, while in salty water our body is lighter then all the salt in the water allowing us to float better.
I hope this made sense and I hope it helped! Let me know!