bouyancy and weight are different things, and you are confusung them. read the description of any large ship, and you will find a reference to it's "displacement", the amount of water it would push aside. this is much larger than the ship's actual weight.
i once saw a science experiment: a gadget called a "drip tank" was hooked up. it was perfectly full of water, so if something was dropped into it, the same volume of water would "drip" into a nearby bucket.
the teacher then slowly dropped a large piece of clay shaped like a brick into the tank. it pushed out a quantity of water that weighed about as much as the clay.
then he took a second, identical piece of clay, molded it into a crude boat, and slowly dropped it into the tank. result: it pushed out 3 or 4 times as much water, and floated!