1) You don't need to wear a wetsuit. Although others will look at you like you are crazy. I have never worn a wetsuit, mostly because I didn't want to shell out the money or deal with peeling it off. However, I have heard that wetsuits do help with your buoyance and keep in mind I haven't participated in any swims with extremely cold water temperatures.
2) Wear the proper equipment. Caps will keep your head warmer and cut down on drag. Goggles will help you see better. I also consider my road id to be an important piece of equipment. If something were to happen to me in the water there is no identification on to help rescuers (I actually consider this important for all athletic events or trainings that I participate in).
3) If you are intimidated by the stories about getting hit or kicked look for swims that start in deep water. If you start in deep water then everyone already has some space around them from treading water and there is less collisions. The swims that involve the most bumps are those when you run into the water, at least in my experience.
4) If you are doing a race where you start out on land and then run into the water prepare mentally for the shock of the water. I have found that the water always feels colder than I expected. Once you run into the water stay calm, take deep breaths and know that you can always breath with your head out of water or flip onto your back if needed.
5) Most the other swimmers are swimming in the middle. It is okay to move over to one side or the other or to drop back so you can swim your race. Better to be a bit back or to the side in the beginning and swim with a good stroke, than in the middle, but not swimming efficiently.
6) Train with sighting. If you do most of your swimming in the pool then you don't need to sight for the finish line or to stay straight. Unfortunately, this is a big part of open water swims. There are two reasons to practice sighting. One, this will build up your muscles. My neck is always sore after my open water swims, the more you practice the less sore you will be the next day. Two, swimming while sighting drops your hips and can effect your breathing so you want to practice this before you are actually racing.
7) I breath bilaterally, on both sides. This is definitely helpful because you can see those around you better and you don't have to sight as much. The less often you breath the faster you will be, but more importantly breathing on every odd stroke will allow you to even out your stroke and swim straighter. If you are new to breathing bilaterally you can start out with the pattern of breathing twice on one side, stroking for three strokes and then breathing twice on the other side. I actually tend to breath this pattern while extending the amount of strokes between the breathing.
8) Swim as far as you can at the finish. It is much easier to swim through deep water than to run. Also, the water can be an amplifier, so stopping to stand up and then not reaching the bottom will slow down your finish. I swim until I am unable to complete my pull correctly and my fingers are scraping against the bottom. I have actually passed a few other swimmers at the finish using this technique.
9) Have fun!