well, they are closely related due to the fact that most of the dissolved solids in seawater are salts. but, other things can also attribute to salinity. specific gravity is the ratio of the density of a liquid to the density of water and is expressed as a dimensionless number, such as 1.023. this means that the sample is 1.023 times as dense as pure water, and assuming no other contributors, the difference is attributable to salt. hope this helps...