either way, surfer and his board have been stated as being made of the same material. you take something with the same chemical makeup and somehow make it denser, it is no longer that materiel.
how about graphite and dimonds, if i had a stick of graphite to use in a pencil and a dimond in a ring, would you say they are the same material, hellz no! If you say yes, try convincing your girlfriend that next christmas. They are after all both made of pure carbon right?
the reason dimonds are denser is not because they are denser graphite. with that logic, you could simply crush graphite into dimonds. If you were to atempt that, the graphite would simply fall apart. Dimonds are dense because of how they are formed in the first place, or rather because of how the molecular structure is arranged. If you have a different molecular structure, you have a different material.