It isn't possible to see the molecules of an object without special equipment but knowing the density of an object helps us predict what we can't see if water has a density of 1kg/m and The molecules look like the image below describe what the molecules might look like for the samples on either side compared to water molecules in lead will be

F
g

=M
sub

⋅gF, start subscript, g, end subscript, equals, M, start subscript, s, u, b, end subscript, dot, g
where g is the acceleration due to gravity, 9.8 m/s/s. But you know that gravity isn’t the only force that acts on the submarine. There must be a buoyant force that acts to counteract it. The submarine is filled with air (like a balloon), so you expect it would float on top of the water were it not for the action of its propeller: even though the steel walls of the submarine make it heavy, this added weight is offset by a huge amount of trapped air. This effect is similar to a balloon, which doesn’t float until it’s filled with air. The submarine thus requires a propeller that acts together with gravity to make the submarine sink, despite the buoyant force acting on it.