Maintaining Your Composure
In almost every pool I know, it is impossible to be more than 4 lane widths from a wall. And with lane ropes installed, there is always something within an arms reach for support in case you get a mouth full of water. That is not the case with open water.
Swimming alone or with a small group, there is often nothing right next to you to hang onto. At least in races, there are usually lifeguards on rescue boards or kayaks nearby to quickly lend assistance. How far do you feel comfortable swimming without hanging onto something?
Think carefully about that question, because the answer is very important. In the pool, do you grab the wall at each turn in order to get a little rest or a little more breath? Can you swim longer distances without grabbing a wall? Can you stay afloat while coughing from getting a mouthful of water, or do you hang onto the lane rope? Can you keep swimming when you get a side stitch, calf cramp, or foot cramp? Can you swim underwater for a few seconds without feeling claustrophobic?
Although it is not uncommon for even very experienced swimmers to occasionally feel a little panicky in open water, panicking is about the worst thing you can do in open water (it isn't recommended in many other situations either). The biggest mental challenge to open water swimming is to maintain composure no matter what happens. This may require doing some breaststroke, or even treading water or floating for a little while. I have always valued my life a lot more than staying with the pack, a fast time, or riding a great wave. Find some way to keep afloat and to regain your composure. With this mental security, you will at least enable yourself to enter another event at a later date. With that in mind, I am not aware of any open water races that require you to swim any particular stroke, so do whatever stroke you feel most comfortable with.
outdoor sports / military survival