Blowing a fan into the sail does work; but it's nowhere near as efficient as normal airboats. You're not gonna move very fast, and it takes a helluva lot of force just to go like 1/16 the speed you could if you just point the fan behind you.

The magnet thing might work the same way; but you'd need a super powerful magnet. Like probably an electromagnet. And at that point, an electric engine is just a more efficient use of the energy.