How could you compute the density of the human body?

Fill a bath or other suitably sized vessel with a known volume of water. Mark the surface of this volume. Submerge the human subject, and mark the new level as well. Remove the human, ensuring as much water as possible is returned to the vessel. Now fill the vessel with water, measuring the amount used, until it matches the level of the submerged person. Weigh the person and determine their mass, then divide this by the volume you used to match the second level. There are many variations on this and all have widely varying degrees of accuracy, but all involve determining the mass and the volum, then dividing the two. Note that this is only the density of one human; you would need to obtain a suitable demographic sample size, test all of them and calculate the average to state that it is the density of the human body.