if the crack is on the rail, put a small piece of cloth on for support. a lot of the board's structural strength is in the rails. all the surf shops have epoxy repair kits. after mixing the 2 parts per the directions, it's just like working with poly, except it takes a full 24 hours to cure. and it smells better, too.
JMD probably missed the part about all the little holes.
It may still resist water, but you want to maintain it's structural integrity. If you can see foam, patch with cloth. Sand the area with 150 grit. Cut a patch about an inch bigger than the crack all around. Mix up the epoxy. Apply the patch with the resin making sure to soak the whole patch completely with resin. Let cure. Sand excess resin off and smooth, but don't sand through the patch. Mix more epoxy. Re-apply resin over patch with a chip paint brush for a smooth coat. Sand smooth with 150, 220, 320, 400 grits. Good as new.
If the crack is more like a spider crack then I would just sand the crack, apply some resin to seal it up. Then finish sand as described above. Done. With a spider crack the fibers are still intact for the most part and you still have some strength so just sealing with resin is really enough. Good luck.