Jackie – it’s all to do with how much water is displaced. The average weight of the boat (including all the air inside) is less than the weight of the water that gets pushed aside as the boat settles into the water. In fact, archimedes found out that the mass of the water that gets pushed aside is exactly equal to the mass of the boat. If the boat couldn’t push aside enough water to match its mass before it was totally submerged, it would sink.
A steel bar on average weighs a lot more than water, so it doesn’t have a hope of displacing its own mass worth of water.