Salinity and Density

Source: Why is the ocean salty? | NOAA

Some areas of the ocean are more salty than others. This image shows methane mussels living at the edge of an underwater brine pool in a cavern at a depth of 650 feet in the Gulf of Mexico. The pool of brine is nearly four times as salty as seawater. Salty water is more dense (heavier) than freshwater because it has salt ions between the water molecules. This picture was taken by a submarine which was floating entirely on top of the pool due to the extreme density of the salt water.