The strength of this unit is really the
chance it gives students to explore this concept for themselves. A
brief, fairly clear explanation of buoyancy can be found at the UnMuseum, another, with a couple of challenging brain teasers is part of NOVA's Voyage of Doom.
Since students make "boats" from clay, why not extend the unit with learning something about the history of boats?
One of the oldest types of boats on earth is the coracle. Here is a picture of a coracle in use in Wales. Native people in North America made a similar vessel called a bullboat.
Other early boat types include the dugout canoe.
Many students build boats shaped like barges, a little like the Waterfront Museum's historic barge in Brooklyn, New York.