As you add dissolved solids to pure water to increase the salinity, the density increases. As you add salts to seawater, you also change some other properties. Increasing salinity increases the boiling point and decreases the freezing point. Increasing salinity also lowers the temperature of maximum density. When Water Freezes, It Expands Fig.
Water expands when it freezes making it less dense than the water from which it freezes.
Once the oceans filled with ice, life there would not be possible. We are all aware that expansion of liquid water to ice exerts a tremendous force.