Polish = good looking.
600 grade wet n dry all over = fast. It was proven long long ago that a slightly rough surface is faster.
Windsurfers, yachts, surfers etc went thru all that in the early 1990's.
Poddy, if you reckon car polish will screw up a SUP, I feel sorry for your car. What they hell do you rub it with?

Most polishes nowadays are nothing but silicone with no abrasive. A mega fine cut polish and buffing wheel is what made your new car look even better than your SUP and that is an abrasive polish.