Big aircraft carriers weigh in at 110 thousand tons. I do not know of anything heavier.
Jahre Viking, crude oil tanker
The Nimitz Class U.S. Air Craft Carrier
They stay afloat the same way any ship stays afloat. The displacement of the vessel has to be less then the amount of water being displaced. The armaments have to be carefully planned to make sure that things don't get too heavy. And the balance and location have to considered as well, to make sure the ship doesn't roll over.
Simply put, White Star Lines had the largest ship afloat constructed for them, and they outfitted it with nothing but the best. That's why Titanic was so expensive.
The bouyancy of the vessel keeps it afloat. As long as the force of bouyancy is larger then the mass of the ship it will stay afloat.
When the ship sank I used a lifebelt to help me keep afloat.
The upwards thrust pushing on the hull keeps it afloat. As long as the hull is the right shape then it will always float no matter how heavy it is!
The ship was designed and built to stay afloat if four watertight compartments were breached, but the iceberg breached five compartments, so the ship couldn't stay afloat
The superstructure is the part of a ship above the main deck, carried by the ship but not responsible for keeping it afloat.
Because the water pushes the boat up and the weight pushes the boat down so that it stay afloat
Inside NCIS - 2012 Ship Set Set Afloat 1-5 was released on: USA: 12 December 2012
The ship has enough gas in it to keep it afloat.