Just in case anyone was wondering:

Butter is less dense than water, because it is oil. Butter has a density of 911 g/L, against water's density of 1000 g/L. This gives each liter of butter 89 g/l of extra weight it can support in water before it is more dense than water. Due to this, you could build a flat raft out of butter. To float a 140 pound human (me) you would need 180 pounds of butter. If you stacked the 1 pound bricks 4 thick, you could make a raft that was about 12.2 feet long by 5.6 feet long by 1 foot thick. In total, this project would cost about 1200 US dollars, not counting whatever you might use to hold all the butter together.

Hmm . . . Maybe if you sort of drove rods through all of them, to connect each line of potatoes like a big kebab? You'd have to use something with the same density as water, though.

True. The only thing is, the material has to be the same density of water, because the whole point of the project is that the raft floats by either the butter or potatoes entirely. If you used wood to hold the potatoes together, the wood would help the raft float, which defeats the purpose.

@LBPHacker: True.