Acclimating to the cold takes time, along with a heavy dose of patience. If you know you'll be racing in a chillier climate, it is smart to begin training in cold conditions long before the race. However, there are race scenarios when the weather is throwing a fit, and it is significantly colder on race morning than expected.
If warming up in the water isn't an option, you still need to warm up your body and raise your heart rate before you toe the line. Go for an easy 10-minute jog, finishing just before your wave start. This will create some internal heat—just enough to look forward to some "refreshing" water and dull the initial shock of the temperature.
When the gun goes off, rather than sprinting into the water and being shocked by the temperature, walk in slowly to gradually acclimate and allow your breath to adjust more easily. Focus exclusively on relaxing your breath with a smooth inhale, followed by a relaxed sighing exhale. For the first few hundred yards, try to remain exclusively focused on your breath; in your head say, "inhale, exhale, inhale, exhale." Going through this mantra will help block out all other noise and negative self-talk that can become a distraction.