A sea can also be very warm for most of the year. The Red Sea, for instance, has an average temperature of around 30 degrees Celsius (86 degrees Fahrenheit). It is also the saltiest sea, containing 41 parts of salt per 1,000 parts of seawater. Seas can be quite cold, too.The Greenland Sea, for instance, has surface water that hovers near the freezing mark most of the year.