The average ocean salinity is 35 ppt. This number does vary, usually between 32 and 37 ppt. Rainfall, evaporation, river runoff and ice formation cause the variations. For example, the Black Sea is so diluted by river runoff, its average salinity is only 16 ppt.

Freshwater salinity is usually less than 0.5 ppt. Water between 0.5 ppt and 17 ppt is called brackish. Estuaries (where fresh river water meets salty ocean water) are frequent examples of brackish waters.

Animals that live in salty water have developed ways to deal with the salty environment. Most marine creatures keep the salinity inside their bodies at about the same concentration as the water outside their bodies.

If an animal that usually lives in an area with a 35 ppt salinity were to be placed in an environment with a 0.5 ppt salinity, what do you think would happen? The animal would die because of osmosis. Osmosis is the process where water flows from a low salinity environment through a membrane to a higher salinity environment to try and equal out the salt content on both sides of the membrane. In this example, water would flow through the creature's skin (membrane) and into its body, making it swell and eventually die. If this same creature were placed in a very salty environment with salinity higher than 35 ppt, all the water in its body would flow out through the skin, and it would shrivel up and die.

Information provided by: http://www.onr.navy.mil