If you find swimming in the sea far easier than swimming in a freshwater lake, you might wonder: why is it easier to float in salt water than fresh water? Salt water is easier to float in because it is denser. Water with salt in it weights more than water with no salt in it, and the denser the water is, the easier an object will float in it.
Objects float in water by displacing an amount of water that is equal to their own weight. Saltwater is denser than fresh water, so less of it is displaced by the object floating in it. Because it takes less water to equal your body weight, you automatically float higher in salt water than you would in fresh water.
If you want a more in depth explanation, you will need to dig out your old school physics text books for the technical theories relating to the question: why is it easier to float in salt water. The answer can be found in the chapter all about Sir Isaac Newton.
In The Third Law of Motion, Sir Isaac Newton discovered that an object floats because “the mutual forces of action and reaction between two bodies are equal, opposite and collinear”. In simple terms, the object floating in the water exerts a downwards force due to its weight, but an equal force is exerted upwards and this is known as a buoyant force.
When Archimedes sat in his bath and slopped water all over the floor, he soon worked out that the buoyant force is equal to the amount of water displaced. On average, sea water is around 3% denser than fresh water, although this will vary between different bodies of water. Because salt water has more mass per unit volume compared to fresh water, less of it needs to be displaced to exert a buoyant force sufficient to balance your body in the water.
The higher the salt content of the water, the denser it is, and therefore the easier it will be to float. This is why the world famous Dead Sea is well known as being a body of water where you can simply lie there and float forever with no effort whatsoever. Compared to normal sea water, the Dead Sea has considerably more salt in it and is extremely dense.
The Dead Sea is one of the world’s most salty bodies of water and has been measured at 33.7% salinity. Only a few other hypersaline lakes contain higher levels of salinity. This might make floating in it exceptionally easy, but it the surrounding environment is extremely harsh and animals cannot flourish there—which is one of the main reasons why the Dead Sea has derived its name. The Dead Sea has a density of 1.24 kg/liter. This means that although floating in it is pretty much effortless, swimming in the Dead Sea is surprisingly difficult.