Usually pool swims have each individual start from a push in lane 1 and snake (or double snake, depending on distance) leaving xx seconds appart. Open water swims will have either 1 big start from a buoy, dock, or beach or have waves broken out by age group or entry time. When I did Escape from Alcatraz, they just had all of us on a barge and blew the wistle for everyone to start jumping off. That was interesting.
As for the body position discussion, there are several schools of thought, but all involve minimizing resistance. I have read some of the total immersion stuff before, and Spicewood is right on about most world class swimmers spending most of their time on their side. From my experience and most of my coaching, body rotation is very important. One concept that was taught to me years ago and I use to teach new swimmers is to think of a pole going straight through your spine that your entire body rotates on. So, your shoulders and hips rotate together as you reach forward each stroke. This should also keep your head stationary and up (with the water hitting roughly at your hairline). Hope that helps a little.