Like northsea said, you do need to clean up any loose bits first. If I remember correctly, you are sailing a Nacra? The cores they used in older foils are a solid resin based substance and you won't need to do anything weird with it - water migration is not a serious concern. You are also at the tip of the foil where there is not much stress to worry about (other than future hits against the bottom). I would just clean out any loose pieces, fill it, and sand it fair. It will hit bottom again sooner or later and need more attention again.