There are basically 2 types of fears: rational and irrational. Keeping this in the realm of swimming, an example of a rational fear is you might not have the fitness required to get to the swim finish. Of course, as you stand waiting for the swim start, you can review all the workouts you did leading up to the event but, you will never really know if you can finish until you do!
Rather than rational fears, however, most of us fill our heads with irrational fears such as: "there is something under the water that will eat me", or "I am going to drown", or "the last time I did this, I couldn't breathe". There is nothing that will eat you, including sharks (check out the statistics on annual shark attacks). Although there is always the possibility of a drowning, the race directors take every precaution to prevent harm to the athletes (once again, check the statistics). And, my guess is you couldn't breathe because you experienced an anxiety attack, which was probably caused by the irrational fear.
Preparation can help with fears:
1. make sure you swim 2-3x per week and swim in open water often
2. if you are going to use a wetsuit, make sure you purchase it well ahead of event and practice open water swims wearing it
3. check out the swim venue before your event, even if it is the morning of the event. Check buoy placement, swim direction, swim entry and exit, etc.
4. practice deep breaths before the swim start in order to calm your brain and visualize yourself swimming relaxed and confident.
4 x 150yds. pull progressing by 50's to 85%
4 x 50yds. pull progress 1-4 to 90%
4 x 100yds. swim at best interval or rest=5sec.
4 x 75yds. kick progress within the 75 r=10sec.
4 x 25yds. swim max effort r=15sec.
4 x 50yds. done as 25yds. heads up swim/25yds. swim