I also tend to agree with Mark - Just because there's a panning mode on the lens (where IS is turned off on one axis so it won't interfere with the movements) folks automatically assume you need this feature to do panning...
IS can help somewhat, but the fact of the matter is panning does not require IS - more practice may be (Sigma 100-300 f/4 EX with 1.4xTC):
(i.e. "the tip of the board where it was lifting against the water
" is coming from the subjects
an no IS in any lens/camera can detect or compensate for that!)