Congrats on your first triathlon! I myself completed only my second
this past Saturday. I had the exact same experience during my first
tri, I thought I was going to die in the water, literally. I was
wearing a wetsuit that was customized for me (because I am short)had
practiced in both the pool and open water with and without the
wetsuit. The minute I put my head in the water, I was out of breath
and could not catch it. My bouy was non existant and I thought I was
going to drowned. I literally had to float on my back and take a few
deep breaths before I could continue...It took me forever to complete
the 1/2 mile distance but I finished and learned a great deal.
I think you do need to practice in the open water as many have said
but I do think that the anixiety you experienced was just not knowing
what to expect, no one probably told you about the "blender" effect. I
know no one told me.
I have good news though, you can only improve from here, your first
tri is over and you know where you need to improvement. I can
guarantee your next one will be much better.
I shaved 6 minutes off my swim time. (no wetsuit this time) I was much
more confident and knew to stay on the far right which is the side I
breath on and to the back of the pack...(I knew I would be slower and
did not want to get swum over)
Have you heard of total immersion? Check it out, I bought the book and
DVD and it has helped me a great deal.