in here if you drop down directly to the grouper from straight up it will run away much before speargun's range. If you see them in open dive from little away.Preferably dive to the rocks around the grouper and hide yourself behind the rocks, then show yourself little and wait little to check if it will come to aspetto or not, if it don't come and you have plenty oxygen left, hide again and try to swim toward it (not directly) while covering/hiding yourself with rocks. If you can't get close to it after this method it will either run like hell and dissapear or will enter the cave nearby. If it enters the cave you still have chance to find and shoot it in the cave. Make your next dive there
Not an expert here but this generally works
