@GiddYupJoe , I agree with
@Murf'n'surf for the most part. I say for the most part because sometimes the "wander" is because of a little less drag one side or the other, or because you have two pumps turning the same direction and causing the heading drift. You can't however "hold heading" in a jet boat near as easy as something with a rudder. With fins, you should be able to hold "track" much easier, with the wandering accepted as normal. But it sounds to me like you are fighting it.
A quick reminder about what I am sure you already know but just considering all of it again sometimes helps to slow your responses to certain conditions and increase them to others. Because you do not have a true rudder and because it takes "some thrust" to actually turn the boat, you must change the direction of the nozzles to force the back end (stern) of the boat over to get the desired heading. So considering you must have some thrust to move the stern and change a heading, and considering that if your in neutral on your throttles, you are messing up the direction of the thrust completely. All of these boats drift in neutral. No exceptions. It isn't possible to stop them from drifting completely. Because you still have impellers turning and forcing water out the nozzles, even if at idle. Neutral is only the position of the reverse gate hanging partially over the nozzle, and deflecting some water forward and some water rearward. But the water that is blocked by the reverse gate, is actually coming up and hitting under the deck above the nozzles and creating turbulence. It is not possible to really be in neutral. Most of these boats, from the factory, will drift forward slightly in neutral. You have zero steering control in neutral. So if your trying to move in neutral, just because it wants to move forward, and have any directional capability in neutral, your literally swimming upstream.
The best control you will have, is throttles in the first detent forward of neutral, the lowest "no wake mode" setting, and try to keep from turning the wheel. Consider a 10 to 15 second lag from what you input until when you see it in heading change. So any secondary change you make to heading prior to seeing the results of the previous heading change is going to not be seen for the same amount of delay. I may be exaggerating the length of the delay, but it is LONG! Driving these vessels with authority is a "learned skill", not second nature. But if you just take an hour to practice this, it will be a skill you have! Do it on a calm day, open water, and pick a point on the horizon to track to. Put the bow on the point and keep it there with the absolute minimum amount of input to the wheel that you can do. Yes, your going to chase it at times...but that is because you set up the occilation by over controlling. In the flying world...it is PIO (pilot induced occilation). And it is over control. Over control gets bigger and bigger as you get farther behind with your input compared to what the boat is lagging to do. So you literally have to stop because you are making it worse. And getting the wheel centered is the biggest obstacle. Where is center? Well, center the wheel and look at the nozzles when on the trailer. You don't need a straightedge or measure the nozzles. There is so much slack and slop in the nozzles that you don't need to worry with any of that. (different if were talking about a pull at higher speeds) Just put the heading on the horizon or point on shore, and stop chasing to hold it. Practice that. If you have to correct, move the wheel an inch or two and then back to where neutral was. Always return the wheel to a neutral or no turn position. Don't leave it in the turn until you see a turn, and then turn past neutral the other way to stop the turn. Just small corrections and keep the wheel in neutral position most of the time.
Back to the throttles. Don't chase them either in the longer axis boats like a 240/242. Place them in forward idle, not neutral, and move forward. Feathering the throttles between forward idle and neutral is something that can slow your speed, but it also reduces your directional control. Doing this to increase directional control is something that guys started doing to see if they could produce the same result as the "TDE" that Yamaha put on later boats. I would call this feathering of the throttles to enhance control an advanced maneuver. It is more important that you gain confidence and control of the boat in the majority of conditions and situations you face everyday. Tweaking that as you have mastered control is ok, but putting band aids on something you haven't yet mastered won't help you master it. Some guys will tell you to use "no wake mode" to load your boat. Don't do it. You are locking your thrust in a setting that needs to be able to change when you need to change it. Yes, thrust is steering. but when your loading or docking, you MUST be more proactive than just holding a heading. You can't accept hitting something and you have to MAKE IT HAPPEN, not HOPE IT HAPPENS. So deliberate thrusts and wheel positions to correct drift and counteract wind and wave influence is paramount. And those changes will be fluid and not constant. Practice is the key Nathan, and you will be an expert in short order!