How to break a hyperventilation attack
open water training will solve that feeling, especially if you're able to swim with a group.
i've just recently been getting in an OWS each week for the last three weeks, and something i can't get used to yet is the different scale of space. there are no small markers to tell you how fast you're going. so i rush to try and feel as if i'm moving. i think time will help me get over the need to rush...(and by then, I'll want to rush to improve my race times... ;) )
More open water practice will certainly help - especially if you can work on "bumpier" water. I swim in a lake where we start in smoother, calmer stuff and have an option to swim to buoys further out that are almost always in rougher water. AS far as breathing - it will get better with more practice but one thing that helps a lot is to really push ALL the air OUT of your lungs when you are face down and then come up and take a SMALL breath. This is NOT instictive, oxygen debt comes from carb build up not lack of oxygen. Of course just try to relax more. ALL race starts are generally like you describe - faster in the begginging, washing machine effect - you have to get used to getting winded then recover after a hard first 200-500- meter swim. No different than if you are riding your bike on a fast group ride and you just took a pull on the front - you pull back in the draft and recover. Recover by breathing the way I said above, try to relax and lengthen and smoothe out your stroke - by this time you should also be on good feet. Good luck!

I had a short sprint this past weekend and I was wearing my wetsuit and the water wasn't that cold - cool but not frigid (low 60s) but I still found myself getting into that rapid shallow breathing mode and I stayed there for way longer than I wanted. I really struggled with it for almost half the swim (half mile swim) before I finally got my breath to a reasonable and deeper rate.
Mainly, I think the factors that made it worse were:
1. pre-race excitement/jitters
2. being in the "washing machine" in the first 50+ yards, unable to maneuver and get into a freestyle stroke very well because of being packed fairly close.
3. being unfamiliar swimming in choppy water
Are there some good techniques to halt hyperventilation or is it mainly in my head? I was disappointed that I was able to swim so much better in the pool. I had put in a lot of time training in the pool that I didn't feel my progress was reflected in this race.