Question

How  relative density is related with the density of water??

Answer 1

Relative density =$\frac{density of substance}{density of water}$
it is a dimensionless quantity 

Answer 2

Relative density of a substance=(Density of any substance)/(Density of water at 4 degree Celsius)
so, Relative Density is related with the density of water, taken as 1 g/cm³.
Since RD is a ratio , it is a unit less  quantity.
Example=>The RD of Mercury is 13.6 .