It all started with a Greek mathematician named Archimedes who while taking a bath figured out that his body mass determined how much water overflowed his tub. I would suggest you pick up an exercise physiology book by William Katch & Frank Katch or contact an exercise physiologist at your university as this is a learn by demonstration technique.
Hydrostatic weighing is considered by many to be the most accurate means for assessing body fat (fat floats) and muscle mass (muscle sinks) using a skinfold caliper or electrical impedance. However bone density also will affect the results as African Americans generally have the densest bones, Northern Europeans (whites) average and Asian people have the least dense bones.
The water temperature (35 degrees Celsius or 95 degrees Fahrenheit) affects water density, swimsuit (nylon), hydration and fed status of the participant are critical. In addition, the participant must practice exhaling all the air from their lungs prior to being submerged underwater for 5 seconds. (Can you imagine someone telling you to force all the air out of your lungs and then dunk you underwater?) A person’s first inclination when dunking underwater is to take a big breath which would enter an error for hydrostatic weighing. This procedure may be repeated 8 to 12 times depending on the participant’s ability to expel air each time. The difference between an obese and lean individual’s results from hydrostatic weighing is small (0.93 and 1.10 grams/milliliter) so the procedure must be done accurately and repeated.
The participant’s weight out of the water is determined and then their weight in the water. Air weight is divided by water weight to determine the person’s specific gravity (density). The above numbers are close to butter and round steak respectively.