A scale reads 100N when a piece of aluminum is hanging from it. What does it read when it is lowered so that the aluminum is submerged in water?

It isn't quit that simple. You need to know more about the water. Temp and salt content will alter your reading but always be less than 100n. Bouyancy: Archimedes Principle (http://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/buoy_Archimedes.html)

All you really need to know is how the density ( \rho) of the water compares to the density of the aluminum. Yes, these densities depend on things like temp (to a small extent) - but I suspect the OP has been given values for these in his/her text book.

The weight registered by the scale will be equal to:

\frac {\rho _{ Al} - \rho _{Water}}{\rho_ { Al}} \ \times \ 100N.