I'm a bit confused to the "no brainer" type of question that this is. No offense to the user at all. I'm just trying to figure out what you are exactly asking.
Let me try to answer this. In all types of sports that require a piece of equipment (ski, board etc.) that allows the athlete to move across a surface (water, snow etc.), the bottom of this piece of equipment has to be smooth.
If you look at a snow ski or a snow board, the bottom of these pieces of equipment have to be waxed constantly to provide the smoothest bottom. The smoothness allows the board/ski to glide over the snow with no friction. If there were scratches or dents on the underside, this would cause friction and would result in a slower ride.
This also is the same for water sports. If water skis had dents, bumps, scratches etc on the underside, this would result in friction against the water. You want the water to flow underneath the skis/board with no resistance. If you had a scratch or bump on the underside, the water would contact that bump, making you're ride slow with a lot of drag.