If it makes you feel any better, I had a similar episode in my first race of last season. Only thing is...it was my 6th year in the sport!
I've done several Ironman and 1/2IM races, and numerous shorter races. And, yet, after all of that, starting into my 6th season of triathlon, I suffered a hyperventilation problem in the swim at a rather low key Oly. I ran into the water (beach time-trial, start), did a couple dolphin dives, started into my freestyle stroke, put my face in the water, and...I couldn't breathe! It took me until after the turn around buoy before I was finally able to relax enough to swim normally.
I think in my case it was a combination of insufficient acclimatization to the cold water, and that sprint into the water. The sprint into the water, and the normal pre-race anxiety, worked together to shoot my HR sky high right off the bat. And, especially in cold water, I normally try to make very sure I get myself in the water well before the race start to do some "warm up" swimming for 10-15 minutes, to help my body adapt to the cold water. Well, I was dilly-dallying setting up my transition area that day and failed to get down to the water early enough to acclimatize.
So, don't be so disappointed. It can happen even to the experienced. From the sounds of it, for you I think firstly you just need to keep at it. Since you were fine for the first couple minutes, it wasn't a cold water shock sort of reaction. Probably more just general anxiety that shot your HR up quickly. The more you practice in open water, the less anxious you'll be about OW swimming in general.
Next time, give yourself some more time just playing around in the water before you actually start swimming. Get your body acclimatized, and just get comfortable being in the water. Then, start swimming pretty much as slow as you can...very slowly building your pace. I think the key is to let both your body and your mind get as comfortable as possible with the environment before you really start working hard.
Have confidence and keep at it. You'll be fine.
-Steve
Edited by Steve in IL 2007-06-25 5:26 PM