You really aren't going to deal with swims that are crowded enough to be much of a bother until you hit iron distance events. Even then, you will only have those issues with 1000+ people starting at the same time. The reason is because the spectrum of speeds is so wide in an iron distance events, that people are trying to fight to a position in the field where they are swimming comfortably. I mean, I've done a mass start with about 300, and after the first 3 or 4 minutes, there was scarcely any contact at all (I mean, you might have some foot tapping, things like that, but nothing too serious). As far as grabbing goes, most people wont do it--here's why. The better you are as a swimmer, the more you have realized how important rhythm and efficiency are. Grabbing somebody, no matter how it's done, is going to stop your rhythm entirely, and change your body position for the worse. As such, most people realize the competitive disincentive to consciously disturb people around them.
My first traithlon ever (about 2 years ago, when I hadn't been in a pool in about half-a decade--let alone in a competitive scenario) I had to park myself behind the really fat guy wearing a snorkel. I just said, "don't let this guy beat you," and during the last 1/10 of a mile, I dropped him. I was pretty happy I even could muster enough strength to finish the 3/4 mile swim--a distance I don't recommend for your first triathlon. ...I also learned it's not good to attempt the full distance of the triathlon the day before the actual event. I later learned that, too, was an error in judgement.
The cool part about starting in the back? You are going to pass people...I guarantee it. It's a good way to build your confidence up during your first open water tri.