John Dreher is, of course, right about the hazards of hydrogen. It burns very hot, the flame is invisible in a normally lit room, and it is easy to ignite. On the other hand, I know that the web has hundreds of sites that tell you how to make hydrogen, so you don't need Science Buddies for that. I'll limit myself to discussing your questions about the hovercraft.
If you want to use the low density of hydrogen gas to lift the craft, that would be a blimp. The gas would need to be under just atmospheric pressure to be light. If you pumped it into a container at higher pressure, the gas would become denser, and the weight of the container would overcome the buoyant effect of the gas.
If you want to propel the craft by expelling the gas under pressure (like a rocket), hydrogen would be a very poor choice because it is so light. The propulsion force would be proportional to the density of the gas. Of course, you could burn the gas in a jet engine. At that idea, I join John Dreher in his safety concerns.
Good luck, WW