Hovercraft from using reverse polarity of magnets?

not unless the ground was magnetized in one uniform polarity

How will the polarity of magnets create a cushion of air to lift it off the ground?

no.

the ground is not magnetic, so the magnets will do nothing.

Even if you had a steel ground, the magnets would only be attracted to the steel, not repelled.

And what does this have to do with a hovercraft, which uses a cushion of air to provide support?

.

You can make something hover in the air in a fixed position by using a pair of magnets. One magnet with, say, the north pole facing upward is placed on a base. The second magnet, with the same - in this case, north - pole facing downward is placed in the object that is to hover. You will also need a rod or other vertical support for the hovering object. Otherwise, it will want to shift sideways and will simply fall.

It is easiest if the magnets are hollow cylinders. In that way, the vertical support can pass through the center of both magnets, assuring that the hovering object stays in place.

The magnets need to be fairly strong because the repulsion force must exceed the weight of the magnet plus the weight of the hovering object.

The magnets should also have high coercivity. Coercivity is the resistance to demagnetization. Because the magnets are placed with opposing poles, they are trying to demagnetize each other. Magnets with low coercivity will not last long. Whenever you are done playing with the hovercraft, remove it from the vertical support so that the magnets are no longer in opposition. That will make the magnets last longer.

The best magnets for this type of application would be neodymium-iron-born magnets, also known as neo, NIB, or "rare earth" magnets. The term "rare earth" also applies to samarium-cobalt magnets, which would also work well, but are less common.

Neo magnets are available from science supply stores like Edmund Scientific or from specialty shops like K&J magnetics. Here is a link to K&J's ring magnet page:
http://www.kjmagnetics.com/products.asp?cat=16

Magnets like their RX054, RX068 or RX08X0 would work well. Note that these magnets are marked as "axially magnetized". Do not get any that are marked "diametrically magnetized" as these will not work for what you want to do.

Oh, one other thing. Once you have a pair of neo magnets, handle them carefully, as they are magnetically very strong, but physically fragile. Treat them as if they were made from glass. In particular, never allow the magnets to move unrestrained toward a hard surface and especially never allow them to smack together (it happens easier than you would think).

There is a problem using just 2 magnets, which is that the top magnet will fall sideways unless you constrain it. To get around this, use a top magnet (N pole down) then a bottom magnet (N pole down) then a sheet of pyrolyitic graphite (or bismuth) which is diamagnetic. Then the top magnet attracts the bottom magnet, and the bottom magnet always repels the diamagnetic material. Your bottom magnet will then float in a stable fashion.
You need to adjust the position of the top magnet very carefully, there is only a limited range of positions that will give stable levitation.