Couldn't breathe during the swim, help

When you are practicing can you swim past the 100m mark and keep breathing fine?

If so, chalk it up to pre-race jitters and too much excitement.

Been there and done that.  Did you warm up??  I believe what happened to me was in all the excitement of the start I went out too fast from a cold start, resulting in hyperventilating which fueled the fire of anxiety.  In the past 2 events I've warmed up with a couple hundred meters and had no issues.  If you think about it, when you go out for a run, you will often breath pretty hard around the first quarter mile mark until you're warmed up and in a rhytm.  You don't even notice it usually because you can breath whenever you want to and are kind of used to it.  With swimming, however, breathing isn't nearly as easy or natural.  I also make sure I concentrate on exhaling while my face is in the water.  That seems to help get me into a good rhythm.  It gets better each time.  Good luck.

Sounds like a panic attack which always used to happen to me. Ways I've learned to deal with OWS and the panic that I always used to get during competition are as follows.

1) practice swimming and sighting in open water. However, I always got my panic attacks only on race morning and then not during practice. Maybe let others pass and try your best to relax and just swim. It will come with practice.

2) wearing earplugs has helped me also.

3) I do better with a sleeveless wetsuit compared with a full wetsuit. However, if your wetsuit is new, it tends to constrict your chest a bit more until you use it a bit more.

My first tri I had to swim 800 yards and had panic attack right away. I ended up doing the entire swim doing elementary back stroke.

Glad the rest of the race went well for you.

I can really relate, once it was over excitement and no warm up-starting out too fast, next it was anxiety attack.  Hoping to face the demons this weekend and leave my mind at home while my body does the work!