I find it's the first splash of cold water on the face or the first time you put your head underwater that can trigger it (hyperventilation) for me. If you can get into the water and spend a couple of minutes just putting your face into the water and gently blowing bubbles out underwater, rinse and repeat this can help, follow this with some gentle warm up strokes to try to stop yourself going off at a million miles an hour.
There is definitely something in this, but I can't remember where I heard it. Something to do with the vagus nerve blah I can't remember. But it totally worked for me. I read it's the contact of cold water on your face that provokes the sharp intake of air and increased heart rate. If you dunk your whole body in at once, this can be quite the shock to your respiration. I always splash my face several times first, as I'm standing knee deep in the water, then walk in further and dunk my face, then slowly submerge the rest of my body. Works pretty good most times. Also, earplugs have also helped me, not only by combatting the dizziness but stopping the rush of cold water in my ears. It also blocks out a bit of the noise that can add to my stress levels.
I'm not prepared in the slightest for this race, but it will be what it will be. I am nowhere near a half in my training plan. Actually nowhere near a standard distance in my training plan so this should be fairly interesting.... One positive note is that for those who haven't done the Outlaw before, it is superbly organised, friendly and well-supported. The cycle is pretty pan flat so get down on the aerobars