Seawater is a complex solution of many chemicals. Their concentration is, on average, 33-37 parts per thousand by weight. Ninety percent of this is common table salt, sodium chloride.
Although there may be some local variations, the concentration of chemicals in sea water is remarkably constant throughout the seas, and has remained so for millions of years. This means that sea water is a very consistent, “easy” environment to support life.
Seawater is a dynamic chemical environment that interacts constantly with the land, the air and with living things. Roughly 96.5% is made up of water molecules.
c.o.o.l. fact #1
If all the oceans were to evaporate, the salt left behind would cover the entire planet with a layer of salt that was 50 meters (half a soccer field) thick.
(cool fact taken from the c.o.o.l. web site)
Read more about the oxygen and salinity levels in the sea.