text: how does ocean salt water stay separated from seas or gulfs non-salt water?   sheer volume. when a river flows into the sea, it takes time for the waters to mix. while they are mixing, their salt content will vary. also, various currents exist within the oceans, separated by temperature, which also affect salinity. the whole system is controlled by gravity, wind, and solar energy. very complex, but also quite interesting. you might want to pick up an oceanography text if the subject interests you.