Uh-huh, why do you think people get into tanks for measurements?
Can you send me some graviton in a bottle, I’d like to study it.
Displacement measures volume, not weight. It's rather amusing to watch you and your friends struggle with this elementary school concept.
That and arguing that gravity is an undetectable magic force.
Before we go any further with the displacement thing, let me ask a simple question.
When you say “immersed” do you mean completely immersed, or floating? I’m assuming you mean completely immersed.
It makes no difference to my argument, but I’d like to avoid any obvious sources of misunderstanding.
If you measure the displacement of a floating object you can calculate the object’s weight, because it will be the same as the weight of the water displaced. Or you can measure the volume of water and apply a simple formula, because the weight per volume of water is known.
But to estimate body fat you would have to find the volume of the completely immersed body and compare it with weight. Then apply some rule of thumb formula for body weight vs volume.
But if you measure only the water displaced by a completely immersed body, you know nothing about its weight.