|MadSci Network: Chemistry|
If I'm understanding your question correctly, the simple answer is no.
When you find the desnity of a liquid the equation is D = mass/volume as you stated.
When we compare densities typically it's represented as (# of grams / 1 ml of vol.) or (g/ml)
In order for a substance to be less dense than water, 1 ml of that substance must have a mass of less than 1g.
So mathematically this is impossible.
I hope this is enough to answer your question.
Try the links in the MadSci Library for more information on Chemistry.