Salinity: the amount of dissolved salt in seawater. It is expressed in the unit PPT (parts per thousand) or PSU (practical salinity units). The ocean's average salinity is 35psu, but this value can vary when evaporation increases the salinity, or precipitation decreases this salinity. A rapid change in salinity is called the halocline.
Temperature: the temperature of the ocean decreases with depth, and varies with latitude. Compared with fresh water, sea water freezes at a colder temperature, and because gasses dissolve better in cold water, sea water has a higher oxygen content. A thermocline is a rapid change in temperature.
Density: the amount of material or mass per unit volume. Temperature and salinity determine the density of seawater. Cold water is denser than warm water, and an increase in salinity causes denser water than a decrease in salinity. The rapid change in density is the pycnocline (pictured above).
Pressure: on land the average pressure is 1 atm. Underwater the pressure increases by 1 atm ever 10 meters (33ft), and as pressure increases gasses are compressed. In order to deal with pressure some fish have a swim bladder, in the swim bladder oil replaces air.