The main determining factor on whether the water in a draining toilet flows clockwise or counter-clockwise is the way the water enters the toilet. Water in a toilet is squirted from just under the rim when the toilet is flushed, and the direction that the water comes from determines the direction in which it flows.
There is a misconception that toilets in the northern hemisphere spin opposite to the toilets in the southern hemisphere. This is attributed to the Coriolis force, which governs the movement of water in relation to the earth's spin. The Coriolis force affects large bodies of water; a toilet is far too small to be affected.
For the Coriolis force to exist, the water in question needs to cover a certain amount of space. According to Discovery.com, even a toilet a mile wide may be too small to experience this effect. In nature, the Coriolis force may be halted by friction and changes in the barometric pressure.