Slow and Steady Start Wins the Race.
Many a racer has burned out midway through a race because they started out too quickly. Since the first leg of a triathlon is in the water, plan to train and swim at a steady pace that you can sustain for the entire leg. You should train for the exact strategy you will use in your swimming segment. As you approach the water, you should run until the water reaches knee-level, as this will maximize your efficiency.
In addition, prepare yourself mentally for the psychology of the other racers. Whereas human psychology will prompt the other racers to compete for the pole position, you should stay relaxed and maintain proper breathing. While other racers will try to align themselves as close as possible to the first buoy, you should not follow the pack. Instead, feel comfortable starting outside, and then eventually come into the line as you near the first buoy. By not over-expending energy in the water, you will be refreshed for the cycle.