Seawater is denser than fresh water because of the salts’ added mass.
The freezing point of sea water is about -2 ºC.
Deep in the ocean, under high pressure, seawater can reach a density of 1,050 kg•m−3 or higher.
From wikipedia:
The average density of seawater at the ocean surface is 1.025 g/ml; seawater is denser than freshwater (which reaches a maximum density of 1.000 g/ml at a temperature of 4 °C (39 °F)) because of the salts’ added mass. The freezing point of sea water decreases with increasing salinity and is about −2 °C (28.4 °F) at 35 g/L (equivalent to 599 mM/L).[1] The coldest sea water ever was discovered in a stream under a glacier in the Antarctic in 2010 and measured −2.6 °C (27 °F).