What can be the highest density in seas, rivers and oceans?

The ocean water is most dense. Sea water is less dense, and river water is least.

Pure water has density of 1000kg/m^3.
Ocean water has a lot of salt dissolved in it, so the density increases. The density is between 1027 to 1050 kg/m^3 in ocean.

Seas have lesser dissolved salts. So density is 1020 to 1030 kg/m^3.

Rivers have nearly pure water. So, the density is near to 1000 to 1010 kg/m^3.

So, density is most in oceans, due to salt dissolved in water.

The highest density is seen in dead sea, which is 1240 kg/m^3.