Do molecular substances tend to have LOWER densities than non-molecular substances?
They tend to...........
In a molecular compound, there is little intermolecular force between molecules. Gases, certainly molecular materials, in which the space between molecules is vast, are inherently LOW density materials. On the other hand, for solids (and less so liquids), most of their volume is occupied by close-packed atoms in which all the space is utilized, subject to the requirements of packing, and thus tend to have high densities.
So what do you think? Would a diamond float on water? Remember that diamond is a non-molecular material.