Well, if you aim to go spear fishing, you first get yourself a nice sharp spear, then, while standing in whatever creek, lake or ocean you've selected and see the type of fish you've gone hunting for, you take nice careful aim at that fish and spear it.
Remember light refraction; generally you should aim slightly lower than where you see the fish. Aiming lower should bring your spear closer to the actual target.
When you put the end of your spear into the water it looks like the shaft is kind of bent because the water causes a bending of light rays. The fish isn't where it appears to be from above the water and you have to account for that.