WEIGHT AND MASS

I would suggest that you would need the displacement (lowering someone into a tank of water will displace a certain measurable quantity of water). Then you would need the weight. Those two figures will give you what you need to determine the density. This is the classic answer which Archimedes (allegedly) came up with as he ran naked through the streets of Syracuse shouting "Eureka"--he had climbed into his bath, sloshing water out of the tub, and he realized how he could solve the problem of quickly and easily testing objects made of gold to make sure they were pure. It is also often expressed as the specific gravity of the object or substance being measured, by comparing the density to water.