Sea water has a higher concentration of salt. Therefore, its density (mass per unit volume) is higher than that of fresh water. That's why we float more easily in sea water than in fresh water. Furthermore, that's why people can barely sink in the Dead Sea, which is a sea with an unusually high salt concentration (i.e., an incredibly high density). This difference in density between fresh water and salt water is what causes the apparent separation between the two types of water when the waters of a river meet the waters of a sea. However, if you were to dive below the surface of the waters you would witness a great amount of turbulence. This turbulence is caused by the mixing of the waters and was described as early as the 1st century AD by Pliny the Elder, a Roman naturalist, who observed this phenomenon and called it "estuarine circulation". The turbulence takes place when the velocity of the fresh waters equals that of the salt waters, triggering the mixing between the two types of water. To summarize, the waters of the river do mix with the waters of the sea, although this happens below the surface and not at the surface. If you don't believe me, just go to your kitchen and mix salt water with fresh water. You'll see for yourself that they do mix with each other.