Did you know that you mustn't be less dense than a fluid before you can float?
Yes, you mustn't!!
Whenever a body is immersed in liquid, it exerts its own weight on the liquid, taking its own volume and displacing that(same volume) of the liquid that was in its current position. There is a tension within the liquid body as a result of the forces of attraction between the molecules. For this reason, an opposite force pushes the immersed body upwards opposing its weight and fulfilling the action-reaction law(Newton's Third Law of Motion). This upward force is known as upthrust and equals the weight of the liquid displaced.
If this upthrust equals or exceeds the weight of the body at the time of immersion, then it'll float in the liquid, otherwise, it sinks.
In other words, a cork will float on water because the weight of water displaced is greater than that exerted by the cork. This is because the cork is less dense than water I.e at equal volumes, water has more weight. Therefore a body will float in a fluid(water, air, etc...) if it is less dense than the fluid.
So tell me, Why a ship or submarine made of so many metals also containing many humans and cargos, way denser than water, float... Men, its gonna sink!!!!
This is how cool science is, coming up brilliant means of solving man's problems. Why don't you show science some love!!
Density isn't an only factor for floatation but there's also the "shape" of the immersed body.
This is the factor applied in a ship's and submarine's designs.
I want you to try this. Take a flat aluminum foil and place it on water, what happens? It floats.
But aluminum is 2.7 times denser than water(2.7g/cm3)
Alright, again take the same aluminum foil and wrap it like a ball and place it on water, what happens now? It sinks!!
You discover that when a denser body is shaped in such a way that a larger volume of it can come in contact with the fluid, it floats. Because it has displaced a large volume of the fluid. In the case of the wrapped foil, at the time of immersion, a small volume was displaced and it eventually sank. But it was a different case when it was flat.
The ship, therefore, floats because it is designed with a large surface volume at the bottom in order to displace a large volume of water, thus providing a big upthrust.
Swimmers also do this, by placing their body on the water in a way that creates a big upthrust and allows them to float.
To wrap it up, floatation isn't just an aggregate of density but also of shape!!