Quite forward of you, Shane. Compared to the ship, you are very dense. But, please, don't take offense. We're not talking about your mental acuity here. Density, in this case, is a function of weight divided by volume.
Every substance has density, including water.
If the density of a given object is greater than the density of the water around it, it sinks. If it is less, it floats. Simple as that. That supertanker may be made of material that is denser than water, and it may outweigh you by a few thousand tons, but much of the area enclosed by the ship's metal hull is just air. Eureka' There is a law of physics that covers this.