Is this the time someone should shout "EUREKA"?
I might be totally wrong but I'm not sure the weight of water displaced exactly equals the weight of the boat. Some boat specs state a weight and also a displacement, which are normally different. I'm trying to think back to O Level physics, but I think the displacement is more about measuring volume isn't it?
For example, a cube of lead will weigh different from a cube of gold the same size in the air. However, drop them in water, they will displace exactly the same volumn of water if they are the same physical size as they will both sink.