When I saw the moment,my brain remainded me the concept of buoyancy.I find such a good real life example of buoyancy and how they apply the concept here!These two empty container were floating easily while the iron made instrument was almost drowning into the water.These boatman bind the iron instrument with these containers so that it can atleast partially get some help for floating.These containers also decrease the weight of iron instrument. Bcoz, buoyancy always works at the upper direction.