People who like adventure. People who just want to get out and see beautiful places. For some it will click, while others will move onto other sports. People looking for a new and very unique experience, but not the indecisive. Whitewater takes instantaneous decision making abilities.
Hesitation is failure.
This doesn't mean death. On class I it just means you blew the move-no big deal. In class III, you look like a fool as you spend time upside down. In class V you die. Either way, you have to be able to react without thinking, and that reaction needs to be the right one.