How does the composition of liquid water found deep beneath the Martian surface fundamentally differ from Earth's surface oceans?

The Martian water is a hyper-saline brine, containing extreme concentrations of dissolved salts.

The liquid water existing deep within Mars contrasts sharply with the water covering approximately 71% of Earth's surface. Earth's oceans are predominantly relatively pure H2O, liquid between standard temperature and pressure limits. In contrast, the Martian subsurface reservoirs must contain extreme concentrations of dissolved salts to keep the water from freezing in the cold environment. This makes the Martian liquid highly saline, classified as brine, which drastically alters its chemistry and the potential adaptations required for any hypothetical life within it.