This is because saltwater is denser than freshwater. Density is a measure of how much mass an object has for its size. The more dense an object is, the more it weighs for its size. Saltwater is more dense than freshwater because it contains dissolved salts, such as sodium chloride (table salt). These salts make the water heavier, so it exerts more buoyant force on objects floating in it.
The buoyant force is the upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. The greater the density of the fluid, the greater the buoyant force it exerts. So, since saltwater is denser than freshwater, it exerts a greater buoyant force and makes you more buoyant.
This is why it is easier to float in the ocean than in a lake or river. If you weigh 150 pounds, you will displace 150 pounds of water when you are floating in either saltwater or freshwater. However, because saltwater is denser, it will exert a greater buoyant force on you, and you will float higher in the water.
The difference in buoyancy between saltwater and freshwater is not huge, but it is noticeable. If you are used to floating in freshwater, you may be surprised at how much easier it is to float in the ocean.