Open water swimming is VERY different than swimming in the pool.
First, there's no little black line to follow, so sighting becomes important. To avoid zig-zagging during the whole swim, and thus swimming twice as far as you need to, you need to look up and sight on the buoys every 8-12 strokes or so. You can either practice a bunch and learn to look up on occassion right after you take a breath, or can simply stop and do the breast stroke for a few seconds while you get your bearings.
A trick I learned from one of the better local triathletes is to look up twice in quick succession, right after taking a breath: 1st time is to get the big picture and get a general idea of where the buoy is, 2nd time is to sight in on the buoy. The more straight you can swim, the shorter the distance = faster with less effort.
Second, even if there was a little black line to follow, you probably couldn't see it anyway, because the water clarity usually sucks, which is a technical term by the way. It definitely took me a while getting used to putting my face into water that I couldn't see in.
Third, I'd like to introduce a concept called positive pessimism. I learned this from someone who was an Everest expidition leader. Up on Everest, they used positive pessimism often, e.g. the climbing ropes are very old, but at least they're not very well maintained. How does this apply to open water swimming you ask?
Well, you can't see much and don't know where you're going, but at least it's really crowded and there's lots of other people around bumping into you. Put differently, open water swimming is a full contact sport.
Now that I've probably scared the hell out of you, here are some solutions:
1. Practice!!!!!! If there are any open water swims near you, try going at least a couple of times a month. If not, one alternative is to swim in the pool with your eyes closed (just remember to open them every so often so you don't hit the end of the pool).
2. Self seed in a race. Unless you're a really strong swimmer, there'e no need to get caught in the middle of the washing machine and have people unintentionally beating the hell out of you and swimming over you. Simply start towards the back and outside of the pack and you can avoid most of the melee.
3. If someone elbows you or kicks you, elbow or kick them back. :-)