Here endith the lesson…
Jeezus, I cannot believe I am even responding to this, but denatured alcohol is mainly ethanol with some other additives which make it toxic, so it cannot be consumed (and therefore, is not taxed as booze).
While you are correct that ethanol is, indeed, less dense than water, it is also miscible in water (as are methanol and isopropanol, both common alcohols), meaning that ethanol and water can be mixed in any proportion. So no, it will not float on top of the water. If it did, you would have to shake up the liquor bottles in your cabinet as they are roughly 40:60 ethanol:water.
Thank you for your lesson, though, Professor!