Final answer:
Underwater weighing is a technique used to measure water weight gained by comparing a person's weight on land to their weight underwater. Bathroom scales, conversely, measure weight based on the compression of springs and show mass in kilograms or pounds, which would differ from the Moon due to gravity but not in mass.
Step-by-step explanation:
A method to determine how much water weight someone has gained is by using underwater weighing. This technique requires a person to be weighed on land and then again while submerged in water. Since muscle and bone are denser than water, and fat is less dense, a person with more muscle and bone mass will weigh more when in water. Therefore, if a person's weight in water significantly differs from their weight on land, it could indicate a change in water weight.
Bathroom scales measure an individual's weight by using springs that compress proportionally to the force exerted, representing the person's weight in newtons or pounds. On Earth, scales typically convert this to mass in kilograms, while on the Moon, the weight detected would differ due to lower gravity, but the mass would remain the same.