4.5 Surface water masses
The surface circulation patterns (Figure 15) and the resulting Ekman drift can be compared with the SST (Figure 9) and the sea-surface salinity (Figure 10b). There are large volumes of water in the mixed layers where both the temperature and salinity are relatively constant. Such a water volume with relatively constant temperature and salinity is called a water mass. Section 3 noted that the surface temperature and salinity are determined by the regional climate and so water masses are regionally named (Figure 20).
No, they would not. The density of water is a function of temperature and salinity, and each water mass has a different temperature and salinity value. You can see from Figure 12 that different temperatures and salinities lead to different densities.