People who are not claustrophobic. 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.