The difference here in theory is that the paddler is at the mercy or benefit of the current, no matter what. If he had no visual point of reference in perfectly flat water, he would be mercifully unaware of it's effect. How many things in life may be analogous? The punter on the other hand has the ground as his point of reference. Is he limited to five knots in still water because of his strength or the hull speed of his boat? While the paddler is moving forward with respect to the water at five knots while moving backwards with respect to the ground at 95, it's not clear what effect the current would have on the punter. If he was unable to resist the force of the current, his means of propulsion would totally fail.