The more time you can spend in that 'cold, murky water' before race day, the better. I'm told a cold shower routine can also help conditioning in the early days. Less suprise = more relaxed = fun and games all the way round!
Spend some time just bobbing about in the water before you head off for your training swim (and race infact), also. This really helps your body acclimatize - again, reducing the shock which i imagine is the basis of your claustrophobia. If you can let a bit of water into the suit by pulling the neck, briefly, this will help it settle into place - Bloody cold but once it's done, it's done, and you'll have a much comfier ride as a result.
Best of luck with your event - i've no doubt it'll be the first of many!
Life should be Bisceeezy