Why is it easier to float in salt water?

The short answer is "because salt water is denser than fresh water". That begs the question "so how does that help floating?"

To answer that question, we need to talk about Archimede's Principle. Archimede's Principle determines the strength of buoyancy. When you float in water, there are two main forces at work: gravity pulling you downward and buoyancy, holding you up so you don't sink. To be able to float, the buoyancy has to counteract the gravity. And in salt water, buoyancy is stronger.

Archimede's principle states that the strength of buoyancy is equal to the weight of the water you displaced by being in it. Consider a boat floating on water (forgive my art):

The boat has displaced a certain amount of water. The weight of that volume of water is equal to the buoyancy force that allows the boat to float. The more water the boat displaces, the higher the buoyancy force, because a larger volume of water weighs more. The boat will sink until there is enough of it under water to increase the buoyancy to a level where it can support the boat against gravity.

This, by the way is why boats need large empty spaces in order to be able to float. You want the boat to be able to displace a lot of water, while not being too heavy. Otherwise, even a large buoyancy becomes insufficient to counteract the downward pull of gravity.

Back to our question. How does the higher density of salt water help with the whole floating deal? In Archimede's Principle, what sets the strength of the buoyancy is the weight of the displaced water. Weight can be increased in two ways: 1) having a bigger volume, as discussed above, or 2) being denser. If the water is dense, then a given volume of it will be heavier and therefore, the buoyancy force will be larger. This is why it is easier to float in salt water than in fresh water.

Quiz: Is it easier to float in oil or in water?