If you sink your boat and put it on a scale in the water there would be some reduction in the displayed weight. You would really need to add the weight of the water though because that would count if you tried to lift it on a sling. Of course, the difference is not the weight of the component atoms but, is due to the medium in which the boat is placed. The boat displaces the amount of air or water equal to the volume of it's materials. It's like the old question of which falls faster, a pound of lead or a pound of feathers.

On the other hand, your boat is effectively weightless when it is floating since the boat has displaced a weight of water equal to it's own. Net result=0 and you can quite easily move a 15 ton boat with one hand. That also means that 300 pounds of concrete, whatever it weighs would hold it in position until forces acted on it to move the boat, take up the catenary and then move the block assuming that only the weight of the block was being considered.