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Archimedes sorted this out a long time ago. You displace water when you get into it. The upthrust which you experience is equal to the weight of the water you have displaced. If you are less dense than water, you will sink deeper until the weight you displace equals your own weight - you will float. If you are more dense, even when you are under the water, the weight of displaced water is not enough to overcome your weight; you will sink. It's all a matter of density relative to the fluid in which you are immersed.
But the point I was making is that it is all a matter of density relative to the fluid, until to exceed the density of the fluid. When that point is reached, then any further increase in mass (and thus is density) will not cause any increase in displacement, since you have already sunken to the bottom whatever increase in mass you have. At that point, the only variable that matters is volume (until a further increase in volume causes a sufficient decrease in density as to cause partial floatation).