It's interesting to understand every aspect of this demonstration, within the capabilities of grade 12 students. One very interesting aspect, which involves a little chemistry 12, concerns the question: how do the electrons "flow" through the salt water from one electrode to the other?
This might be a natural question since the force which propels the MHD boat comes from the motion of the SODIUM and CHLORINE ions in the magnetic field and NOT from any moving electrons. In fact, the electrons do not directly travel through the water at all.
In explaining this you need to consider the electro-chemical reactions which occur at each electrode. From a reduction/oxidation table in a standard Chem. 12 text you can come up with the equations for the electrolysis of salt water:
The 2Na+ ions are attracted to negative electrode and combine with the 2OH- ions to form NaOH.
Overall: NaCl(aq) + H2O H2(g) + Cl2(g) + NaOH(aq)
So you can see that electrons are taken up at one electrode and given up at the other electrode to simulate a current flow. Indeed the properties of this behavior approximate the electron flow through wire and any calculations done can be as if the electrons actually do produce a regular circuit.