Iron sinks in water and floats in mercury.
Density of Iron - 7.6g/cm3
Density of Water - 1g/cm3
Density of Mercury - 13.7g/cm3
Weight of 1m3 of sea water is lesser than the weight of fresh water of the same volume.
Density of sea water = 1.3g/cm3
Density of fresh water = 1g/cm3