It’s not the final step though, they’re interested in more.
Why is that so difficult for you to acknowledge?
You brought water displacement up in a discussion of gravity. I don't know why you brought it up. It makes no sense.
Archimedes discovered that water immersion allows you to measure the volume of an irregular solid, such as the human body. A trivial side benefit of finding the volume of a body is that you can weigh the displaced liquid. As Swordmaker points out, if the body floats, the weight of the displaced liquid is the same as the weight of the body.
But if you are not interested in volume, it's a lot easier and less messy just to weigh the body. The reason fat analysis requires the particular methodology of immersion is that volume is needed for the formula.