Fiberglass Hawaii has made a really great video tutorial on how professionals sand surfboards. Even if you are planning on sanding your paddle board with and 5″ orbital sander the steps are the same they just take longer.
The short surfboard in this video is sanded to such a fine grit because this sanding is the final finish. Sanding with 220-grit sandpaper is sufficient if you are going to varnish and the marks are large enough to provide a “tooth” in the surface for varnish to adhere to. The potential problem that can occur is the little swirls from the orbital can show in the wrong light when you stop at 220-grit. You can avoid this potential problem by finishing with a light hand sanding in the direction of the wood’s grain. 220-grit sanding lines are invisible to the naked eye if they are aligned with the grain!