It swirls in whichever direction the water jets are pointing. Sorry, the answer isn't fun, but the whole north south hemisphere thing is a myth.
"The way that the water in your toilet swirls is determined by the way that your toilet is built, not which hemisphere you live in. The toilet shoots the water into the bowl at an angle, which determines which way the water swirls. "
In the case of a drain or bathtub, the force usually described as responsible for the direction the water swirls is a force known as the Coriolis force, which is a force that appears due to the earth's spin. However, this force is not strong enough to affect which direction the water swirls. This is instead due to small things such as how the water falls (if it is a little off center from where the drain is) and the path the water has taken previously. Miniscule things can influence the waters path, and it doesn't really have to do with what hemisphere you're in. Sorry again, it's not as exciting as it seems it should be. Here's another link that explains it maybe better than I can:
The consensus is that hurricanes and such are large enough to be affected by the "Coriolis Force", but things as small as a bathtub or sink are not.
And also, the Earth will not turn into a black hole. It doesn't work that way, and the shape of a galaxy has nothing to do with which direction water swirls in a toilet bowl, or a drain of any sort. Hemispheres also have nothing to do with a magnetic field. The magnetic field goes from the S. pole to the N. pole, so at the equator, the magnetic field would still be going (in the general direction) of the north pole... the spinning of water has nothing to do with the earth's magnetic field.