Re: 100 lb of Lead Vs 100 lb of concrete
This is like "what weights more 100 lb of feathers or 100 lbs of lead." Answer, they weight the same but the feathers (when contained) will float where the lead will sink. It's all about displacement. If a boat displaces 1000 lbs that is based on how much water it displaces at a fixed water density so the water line on the boat would be different in salt water vs fresh water.
So, 100 lbs of lead weighs the same as 100lbs of concrete. But the volume of the concrete would be greater and hence have a bit more bouyancy (wouldn't hold as good as an anchor).