I think the best thing you can do to improve your spear fishing is practice.
Just like in shooting a rifle or a handgun, the more target practice you get, the better you are at hitting the target. With a spear gun, you generally only get one shot. By the time you re-cock, the fish is long gone if you missed, so making a good first shot is critical, but recocking underwater is hard and can be frustrating and a little dangerous in terms of entanglement, so practicing restringing your gun is very important.
When you're sitting down in front of the TV at night, you can practice re-stringing your gun. Of course, don't cock it inside your home (or on a boat for that matter) but you can practice re-stringing it over and over without recocking it. You will be surprised at how easily you get tangled up when you fire your gun for the first time underwater and then try to re-string it and recock it underwater. When there's even a little current, the string becomes hard to manage.
All newbies, and even experienced spearfishermen, in their haste to reload, forget to put the spear back through the bands. Once the spear is in place and restrung, you can't pull the bands over the spear point. If you forgot to load your gun properly in the first place, the bands will be under the gun instead of on top of the shaft. When this happens, you have no choice but to take the spear out and put it back through the bands.
What I do to avoid this is hold the gun upside down, so that the bands are hanging down forming a "O" at the muzzel of the gun. Then put the spear shaft through the "O" formed by the bands as the gun is hanging upside down. By doing it this way, you will always be assured that the bands are in the right place when you're ready to cock.
To practice this technique at home, put your hand in the middle of the gun body and hold the gun in one hand, out from your body so that the gun is upside down and the bands form the "O." The upside down gun and your arm should form the letter "T."
As you're holding the gun with one hand, out from your body and upside down , use the other hand to put the spear butt through the "O" bands, through the muzzle and lock spear in breech by giving it a good tap in place. Put the safety down (the safety on the AB Biller is "on" when it is pointing down -- it is "off" when it is parallel with the spear). With the spear in place, and the gun still upside down, restring it.
With the gun still upside down, the string "catch" will be sticking up right over the trigger. Start looping the string around the trigger "catch" and back over muzzle catch. The rubber part of the string should be the in the last loop that you make because then you can stretch it to make the last loop fit over the trigger catch.
After the gun is strung, turn it over. When you turn the speargun over, the bands should fall in place, ready to be cocked.
I know this sounds complicated, but its not really. If you'll practice this routine until it becomes one fluid motion, you'll save yourself a lot of frustration and get more fish.
If you can find a place where you can practice shooting the gun, like a swimming pool, then you'll be ever further ahead of the learning curve.
