Pure alcohol (ethanol) floats in water because the density of ethanol is fewer than the density of water.
Pure alcohol (ethanol) floats in water because the density of ethanol is less than the density of water.
Pure alcohol (ethanol) floats in water because the density of ethanol is fewer than the density of water.
Pure alcohol (ethanol) floats in water because the density of ethanol is less than the density of water.
For pure numbers, we use "less," and for counts we use "fewer." Density is a pure number in and of itself, not a count of anything else, so we use "less."