The salinity of the open ocean is quite stable in any given area, but
the salinity of shallow inshore waters can fluctuate considerably in
some areas, due primarily to influx of fresh water into the system,
and to a lesser extent, evaporation of fresh water from the system.
Coastal areas near river mouths are apt to have lower salinity then
those more distant from major fresh water sources.  Areas north or
south (east or west) of the fresh water source may be affected
differently, depending on the direction of prevailing currents in the
area.  Also, surface salinity will be temporarily lower after a heavy
rainfall, due to both direct rainfall onto the ocean, and increased
runoff from the land.  Salinity in sheltered bays, inlets, salt
ponds, etc. is often higher than in the nearby ocean, especially
during hot summer weather conditions, due to evaporation of water
from the system.  This is true even in very large enclosed bodies of
salt water, such as the Mediterranen Sea and the Red Sea, which have
a higher salinity than the adjoining ocean waters.  Open ocean
salinity ranges from about 32 to 37 grams of dissolved salts per
kilogram of water.  Some average open ocean salinities for broad
geographic areas are:
Arctic Ocean:  32-33 g/kg
At the equator:  35 g/kg
North Pacific:  34.5 g/kg
South Pacific:  35.5 g/kg
Indian Ocean:  35.5 g/kg
Sargasso Sea:  36.5-37 g/kg
North Atlantic (North Sea):  34 g/kg
South Atlantic:  36-37 g/kg
Gulf of Mexico:  36-37 g/kg
Mediterranean Sea:  37-39 g/kg
Red Sea:  38-41 g/kg
(these values are taken from the Biological Data Book of the
Federation of American Societies For Experimental Biology)
 
Paul M.