If the speed of a longitudinal wave is proportional to the elasticity or compressibility, and inversely proportional to the density, it stands to reason that either must be important variables. Given that the densities differ by about 2.5%, (1000 kg/m^3 vs. 1024 kg/m^3), which is nearly negligible, other factors must come into play that amplify the importance of compressibility (like salinity, Temperature, etc. in sea water). Moreover, the density alone would actually slow sound in sea water as compared to water.
In other words, if you had a question that asked you to identify what best explained these speed differences, you could have the choices:
A. The differences in density between sea and freshwater explain the differences in velocity (WRONG)
B. The differences in refractive indices explain the differences in speed (Not what they're looking for) between the two media.
C. The differences in compressibility explain the differences in speed between the two media (Right)
D. The differences in depth (or something half-convincing)
