Strokes are the bread and butter of many whitewater classes.  In whitewater, we encounter a number of extra factors we don't have in flat water like waves, holes, fast moving water.  You can think of flat water as mostly two dimensional and whitewater is definitely three dimensional (especially in play boats).  Braces become much more important in this environment.  Static draws are really effective and quite useful in ferry moves and edy turns.  Part of three dimensional boating is flipping occasionally so rescue becomes more interesting.  In advanced courses, we concentrate on roll technique for self rescue.