It’s the design of the ship. In order to carry a heavy load the bottom of the ship that is typically submerged must be wide enough so that the ship stays upright. I went to see the ship (the Wasa) that sunk in the Baltic Sea in 1628 and the hull was very narrow. It had been designed to hold a certain weight – for the people on board – but they added many more cannons than planned and it just tipped on its maiden voyage.
Archimedes worked this one out a few years ago. Basically, as long as the mass of the object is less than the mass of water displaced by the object, then it will float. So ships have a big surface area of hull which comes into contact with the water – and this helps to displace large amounts of water.