It has probably already been mentioned, but "knowing your quarry is important". You can read about technique and that is worthwhile, but until you've hunted a certain species, and blown some opportunities you won't really have it dialed.
I'd love to be providing some bluewater experience. Unfortunately, I'm just getting my feet wet in that arena. Hopefully I'll learn more from some of you. I'm especially interested in bluewater techniques that have been successfully practiced in Florida waters, especially Palm Beach County or a neighboring area.
Some of the species I hunt behave differently as they grow larger. One of these is a tasty little reef fish that is so easy to hunt up to about 17" but after that it becomes quite wary. As a smaller fish it will often turn sideways, right in front of you, facilitating the shot. Larger fish of the type will usually rapidly swim away from you though, never even glancing back to avoid presenting a large profile. These large specimens can only be hunted in deep water (on crystal clear days when you can see them from the surface). Generally I'll spot them from a great distance while I'm on the bottom. Then I'll surface and swim directly above it. Then I have to drop down directly on top of them and shoot them in the back (the most difficult shot).
One species I hunt will come in to investigate (provided you are dropping like a falling leaf and not moving much). It will come in only once and it will pass at a minimum distance of about 3 meters. Once it has completed its pass, it will not be seen again. The window of opportunity for a shot on one is perhaps 3 seconds, afterwhich they realize that something is suspicious and rapidly leave. If I miss a specimen under these circumstances, I don't even bother to think about it anymore since there will be no more opportunities.
Some species will come around again and again as a school as you extract one and then another delicious member from it. These are a strong species and I nearly wince after squeezing the trigger on them because if it doesn't hit the spine, things are bound to get crazy.
If you reef hunt, you are probably passing over lots of qualilty game without even seeing them. Fish are experts at avoiding being seen. Focus on a quality hunt rather than covering terrain. A very large fish can be within easy striking distance, observing you, and you will not even know it. Realize that this is not the exception, but rather the rule. The fish are there...but are you savy, and patient enough to see them? On a difficult day I might swim off the reef into the sand, drop to the bottom and dig a little hole to stir things up; sometimes it will make a curious fish reveal itself.
Fish hide in caves and crevices. An area that seems to be devoid of large gamefish may hold lots of good specimens. The key is patience. I have dove down into the same mouth of a cave as many as six times, patiently scanning every single inch of the interior for any sign...and then, in the deep recesses, there it is, just the jutting bottom jaw of a huge specimen who just can't bear the stress of not taking a peek at me. Calm yourself, be patient, get in touch with the beauty of your surroundings, understand the a large fish can be right under your nose without being detected. Look at the same spots over and over again. The fish are there.
Some of my favorite fish will only approach at a relatively long distance and only one time. These are my favorites. By the end of the season these ultra-long shots come so naturally to me that my gun feels like an extension of my body. By the end of the season I rarely miss these arrow-shaped fish that come by at high speed and seem to laughing as if to say "there is no way I can be touched at this speed...YOUCH!"
In general, don't make eye contact with fish and don't chase fish. Unless the species you are pursuing is one of the rare ones that is easier to harvest by being chased, absolutely do not do it. Believe it or not, other fish are watching the show. You are not only scaring the fish you are after, but also other, perhaps higher-quality fish that are nearby. Chasing fish scares all the fish in the area you are hunting, especially the large prized gamefish that are watching your every move. Of course, fish that you have hit, and that have torn off or otherwise gotten off may have to be pursued more aggressively. But even this requires technique. You will be reloading your gun, trying to keep an eye on the fish, swimming, watching for competitors (sharks), etc. Sometimes it is worth a charge to finish a stunned fish off. If however he is getting a sense of himself again, it is often worth it to follow at a distance that puts the fish at some ease, at the edge of visibility. At that point the wounded fish, who wants to rest, will often head under a ledge, giving you time to prepare, huff up properly and execute the kill.
Be careful fighting fish. They are powerful and are in their element. The only thing you have any business doing in very deep water is calmly finning to the surface. Always either have a breakaway so that the spear is tied directly to your surface float, tie your gun to your float and leave it inline between your float and the fish, or have some other means to calmly leave the scene of the shot instead of struggling with a fish at depth. Pull it up from the surface if possible. There is a great likelihood on the reef that the fish will seek refuge in a cave. Buy and keep at least a small SCUBA rig on your boat. Do not come to view this sport such that you accept a free-dive as a necessity, for almost any reason. You do not want to HAVE TO, dive down to extract your fish from the reef. If it is within your abilities to do so, then have at it. But don't struggle on the bottom, burning your last molecule of O2 while struggling with game. Keep a small gaff on your boat for such occurrances. I've had fish look me in the eye at the mouth of a cave, wedged into a crevice, nearly saying "buddy, you are not going to pull me outa here". These are incredibly powerful animals. But when you sink the gaff into the side of their head or elsewhere into their bodies it is remarkable how agreeable they become. Wear gloves at all times and have a knife handy at all times. Understand the anesthetic effect that putting your hand inside the fish's gill plate has. Get your hand inside the gill plate of every fish you shoot at the first available opportunity.
Get in touch with your ability to feel the energy of fish in the water. Often, you can know that quality fish are around before you ever see them. I learned this from a very talented female spearo in the Key West, whose prowess I will never equal. She taught me some of the zen aspects of free-diving and spearfishing. When the season is in full swing, and you have been out a lot, for many hours at a time, you somehow get in sync with the energy of the sea. You see things without your eyes. I cannot explain how to do it, it comes from deep in your soul. I can't count the number of times I've popped my head up and told my son, or other spearos "keep your eyes open, there is something special around here." Inevitably a very large specimen will soon be found or speared, even when the rest of the day has yielded little.
Finally, be careful with your gun. It is a gun in every sense of the word that is capable of easily ending the lives of the people you that you love or horribly disfiguring you and changing the nature of your relatioship with them forever more, in a very terrible way. I have only had one questionable incident, that embarasses me to this day. I laid my gun, with the safety off on the reef and it shot out sideways. My female friend in the keys who was thankfully not in the line of fire, but was on the bottom nearby later said "I'm glad I wasn't in the way of that." I was so embarassed and sickened. The thought of hurting someone with my gun is so dreadful that I can't even think about it. I once hunted with a group of guys I'd only recently met and can't tell you how many times I found their guns accidentally pointed at me. I tried to teach them but they failed to heed my instructions. I will never hunt with them again. Spearguns are firearms. Follow the rules of firearms safety. Don't ever, not even for a fraction of a second, allow your gun to be positioned such that it is ever pointed at another human. I do not have any land guns at this point in my life, but my rule for my house is simple: DO NOT EVER ALLOW THE MUZZLE OF ANY GUN TO BE POINTED AT ANOTHER HUMAN, IRRESPECTIVE OF WHETHER IT IS LOADED. In my household this extends even to a toy gun. I even look at squirtguns as a bad thing to be involved with. Teach yourself and your companions that anything that remotely resembles a gun should be forcefully comanded such that the business end of it never crosses the path of a human. Teach your companions these rules. I have been to meetings of spearfishing clubs and never once heard a lecture on speargun safety. In general people can be taught. It is better if they have traditional firearms experience. In that realm, safety is constantly stressed. If you instruct people who cannot seem to learn though; Do not hunt with them. Do not hunt with people who will not obey these hard and fast rules to the letter. Do not get on their boats, do not allow them on your boat.