I would say that is matter...being at the bottom vs being inside the boat..
At the bottom ..hmmm..the amount displaced may be less than the amount of the water dispaced by the boat with the marble inside..This is just my poor theory ..
Yeah, the volume of the displaced water * density of water = weight of object. Doesn't really matter where it is. If it's in the boat it just adds to the weight of the boat...I'm pretty sure i'm right on this one.