>>105888713>>105889150It's not a density thing, it is a volume and mass. Whether an object floats or sinks in a given medium, like water, depends on whether the mass of the volume of the medium that is displaced by the object weights more, equal to, or less than the weight of the object.
Hot air balloons can float on air because the weight of the air they displace weighs less than the warmer air within the balloon. Same deal if you're talking about blimps. Total weight of the helium used to keep the blimp inflated + the entire blimp itself weighs less than the air it displaces.
Density can be used to calculate the required displacement for buoyancy, but it is not needed. You can just weigh a cubic foot of water, and that tells you all you need to know about how large the boat would have to be to float.
That's why boats are rated in "tons", which refers to the weight of the water they displace and not the actual weight of the vessel itself.