Wow, I've read that 99% of triathletes aren't "swimmers" but I had no idea. I guess I really am lucky. Growing up, swimming was my sport. I kinda fell into tri's after I started spinning. Now, the RUN, that's a tough one.
Seriously though, as a swimmer I wanted to relay my first experience and it helps. My first tri was 400m lake swim. I hadn't trained in open water, although I've swum in lakes many times, just never for sport. As I said, I am a VERY good swimmer. I don't say that to brag but to say that even I was taken off guard by a few things. First, you can't see ANYTHING and the "wave" start totally freaked me out. I hadn't planned on it but I started sprinting just to get away from all the legs and arms. Geez. Then that blasted buoy, which looked really close from the beach, seemed really far away from the water. Not to mention the fact that I was sighting it almost every 3 strokes, when I breathed. It was not what I expected.
My advise is the same as the previous posts. Practice in the open water, even if it's only once. Same with the wetsuit. If you can get several people to simulate the start, even better. I have my first ocean swim tomorrow and I'll be wearing a wetsuit for the first time. I was VERY nervous about the currents and creatures in the ocean but I found a buddy to do a quick swim this week and now I feel 100% better about the ocean and the wetsuit.
Swimming is a very technical sport. It's almost all upper body strength and technique. I've seen very strong men all but drown b/c they thought they could power through the water. Work on your technique. Try to find a good swimmer or coach to help you. Think about it like gears on a bike. One good pedal stroke in a low gear gives you a ton of power on a flat road, while if you use a low gear, on a flat road, you're pedals spin out of control and you go nowhere. A good swimmer can gain a lot of ground with one stroke, a novice will waste a lot of energy and go nowhere.
A proper freestyle stroke, starts by reaching as far out as possible, rotate your shoulder forward and engage the lat. Use the lat to pull the arm out slightly, down and back. The elbow has a slight bend. The hand is closed and cupped but you're using you entire arm to grab the water, not just your hand. Continue to pull through and extend the arm all the way back, really using the triceps, until your thumb brushes your thigh. (you don't actually touch your thigh, although it's a great drill)
I hope I'm not telling you a bunch of stuff you already know but I wanted to offer anything to make your next swim better. The bottom line is that even "swimmers" get anxious in open water. Practice your technique, no surprises on race day and stay calm. Especially in a wet suit. (It's nearly impossible to drown in one)
Good Luck! I'm off to get running advice.