What?
boats float, metal does not: surface area.
Ice always floats, regardless of its form. It floats on water, because it is less dense than water. Believe me.
or because it is comparably dense and solid enough to have a surface area, as well as the surface tension of water.
When water freezes it expands. That is why pipes burst in winter. Thus ice has a larger volume for the same mass of water. Larger volume and same mass equals lower density.
and you do not see a relationship between surface area and density?
There is no relationship between surface area and density. Two objects may have the same mass and density but different surface areas. Consider a cube of modelling putty, if that is reshaped into a sphere the volume and density remain constant but the surface area decreases.