Because more dense has more pressure added to it and if you measered the density you could see that more dense things sink and less dense things float ontop of that water..
no it won't, water was a density of 1g/1cm3 and wood has a density of 0.4g/1cm3 therefore wood can float in water because it is less dense then water. something like concrete (density of 2.3g/1cm3)...