why does the ship cannot sink in water?

Question: why does the ship cannot sink in water?

Asked by 954bmay29 to Ann, David, Doris, Grace, Jonathan, Peter on 2 Jun 2019.
- Grace Kago answered on 2 Jun 2019:
  
  I think that the general rule is that things will sink if they are heavier/more dense than the exact same volume of water?
  
  So, imagine the following scenario: If you put the ship and all the people/equipment in it in a box and weighed it, and then you took the exact same size box and filled it with water, if the ship is lighter, it will stay afloat. In summary, as far as I understand it, the ship will stay afloat if its average density is less than the water around it.

Question: why does the ship cannot sink in water?