This is an easy one. Consider a mooring made from a baloon filled with water, which weight's 300lbs when suspended in air. Submerge it and put a scale under it and what will the scale show - zero because it is the same density as the surrounding water. It's mass is still 300lbs in physics terms but it will not make a very good mooring. You can do an even more extreme experiment - make a mooring from a block of balsa that weights 300lbs when suspended in the air. Try to submerge it and it will want to float because it's density is less that the water.