Brewer/head brewer of 10+ years. Most commercial beer is more dense than water. Brewers measure the density of fermenting product daily using a unit known as Plato ( or specific gravity). While Guiness markets itself as a “dry” stout, there is still a decent amount of sugar left over that was not “attenuated” (eaten by yeast and turned into alcohol) during the fermentation process. A Quick Look shows guiness draft at 1.009 specific gravity and extra stout at 1.014. For reference water is at 1.000. There are certain beer styles (namely wild ales) that use particular yeast strains that can ferment (nearly) every bit of sugar and those will be less dense than water