You have to be a good diver to be a good spearfisherman. With 200 dives under your belt you should have excellent buoyancy control. When you shoot fish you shouldn't be floating up while stringing the fish leaving yourself exposed. You need to stay focused on the fact you are diving with limited gas and time. Do not get so sidetracked in the hunt that you run out and kill yourself. You will be task loading, managing a stringer, gun, sharks, navigation, current, the actual dive, etc. Spearfishing vs. just diving is night and day and so it's best that your diving skills are ingrained muscle memory and automatic.
You have to learn to not make yourself look like a predator. You are already at a disadvantage on scuba. So don't go wild chasing everything on the reef. In fact, often it's best to be 15-20 feet off the bottom to look less like an active predator and keep the fish from easily being able to use their lateral line. When you find your target slowly drop down on it. The fish knows you're there and will often turn broad side to use its lateral line to gauge what and where you're going. This gives you your opportunity. However don't just shoot the fish until you have it in the right angle. Patience is key. The fish should be angled away from you. If it's angled toward you and you shoot it without stoning it, it will swim up your shaft and on to your line creating a pain in the ass for you. If you're freeshafting it will swim right off your shaft and you'll lose the fish. Again, patience is key. It will turn as you approach. Your target on the fish is the lateral line just behind the gill plate. Hit that spot and 9 times out of 10 you'll stone the fish. Jacks and Cobia can kill you. They are strong fish, they can wrap the line around you if you don't spin with the fish. They can knock your mask and reg off. Hell, they can knock teeth out. They can drag you though the water column which is not good on scuba. Always grab every fish by the throat when securing it. String the fish first before taking it off the shaft. If your gun floats, don't let go. Put one arm through the bands before doing your work.
Sharks are a problem, especially on the East coast, but pretty much becoming an issue everywhere around Florida with a substantial increase in population. If a shark shows up, it assumes it's the top predator. Prove it wrong. They are smart animals. They know when you're not looking at it. So keep your eyes on the shark at all times and swim toward it. I repeat, SWIM TOWARDS IT. Doing so will send it off or keep it at bay to make an exit. If the shark does not fear you, you need to be ready to defend yourself. It's unlikely it would attack you if you don't have any fish but they are unpredictable. If one decides it truly is interested in you, unhook your line and be ready to do what you need to do to protect yourself. If two or more sharks show up, just get out of the water. Sending your fish up on a bag is a good idea in sharky waters with someone topside to retrieve it and it's not a bad practice if you have multiple bags, but if you have just one, then obviously you are not going to be very productive.
Personally, I don't care for the trip plan you have described. We live boat, no anchor, no music and are prepared to grab a diver out of the water quickly. It sounds like you're taking a charter and they are going to drop you solo and then move on and do a typical charter with other customers and come back to pick you up. Who's going to get your stringer on an SMB? Is there current? If so are you bringing a float and reel? How long are you going to have to wait until you're picked up? Hanging on the surface with a stringer of fish, floating along the current with the potential for sharks isn't fun. Been there, done that. What if you get injured? Time is of the essence to get a tourniquet on. Can you put one on yourself in the water? Do you have kids? That should play into your risk tolerance decision making.
If these issues haven't been worked out I think you should leave the gun at home and make friends with people who have experience and learn the ropes and do some spearfishing with a buddy. If you're hell bent on shooting there's a couple things to consider. No fish is worth dying for and be a responsible fisherman. Don't shoot short, out of season fish. Most species we target have limits, and underwater they should appear really really big. Don't beat up the reef breaking coral and destroying the environment the fish we want to consume live in. Remember we do this for fun, but more so for food and we want the next generation to have the same opportunity.
Finally, here's a good video you should watch that will expand on my advice.
Watch The Bare Bones of Spearfishing - Your Guide to Underwater Hunting Online | Vimeo On Demand