Definition - What does Water Density mean?
Water Density is defined as the weight of water per unit volume. It is approximately measured as 1 gm/cubic cm. It depends on the water temperature but not in a linear relation or monotonic; instead it is unimodal. Water becomes increasingly dense when cooled from room temperature and reaches the maximum density at about 4 degree Celsius. When cooled further, it expands and become less dense. It is an unusual negative thermal expansion that is attributed to orientation-dependent and strong inter-molecular interactions.
Petropedia explains Water Density
As Water Density varies with temperature, it never contains an absolute density. The density also varies with the type of water, such as fresh water, tap water or salt water. The solid form of water is less dense than the liquid form and thus, an ice block floats on liquid water. On freezing, the water density decreases by 9%. This happens because when it cools, inter-molecular vibrations allow molecules to develop fixed hydrogen bonds with neighbors and lock into positions similar to the hexagonal packing on freezing water to ice.