Different types of catfish eat at different levels of the water column. Some species tend to be more of a scavenger and reside on the bottom. Many fish spend time closer to the bottom (safety, food, etc). Pollution can be found at different levels in the water depending on its makeup.

In my experience, the floating catfish bait is attached to a hook which has a length of line between it and a weight. The fishing line is weighted sufficiently enough to sink it to the bottom. This allows the bait to float up a short distance from the bottom, which allows the fish more opportunity to discover it.