And of course this only works if you’ve got a shape capable of being filled with air under the waterline, right? Hence the post above about how concrete rafts won’t float?
To get a feel for how this principle scales very rapidly into the realms of the enormous…
Imagine an olympic size swimming pool, 50x25x2 metres. The volume of water contained within it weighs 2,500 tons.
This means that if you made a simple, rigid skinned raft of those exact dimensions you would be able to carry a payload somewhere north of 2,000 tons (being generous and allowing a few hundred tons to actually construct the thing.)
So all you now need to do is to increase the depth a little (perhaps from 2m to 10m) and pretty soon you are capable of carrying over 10,000 tons.
Assuming equal dimensions, volume increases as the cube of the dimensions, so a cubic raft of 10m sides has nearly twice the volume of one with 8m sides.