However, in the liquid state the molecules will not form a regular, rigid hexagon. They will bond to each other by hydrogen bonding and pack together. The center of the hexagon in empty space. Water molecules in the solid phase occupies more space. Ice will therefore float on water.
Rivers never become completely frozen because as soon as water freezes, it floats up to the surface.
Fish live in the unfrozen, denser liquid water below the layer of ice.