Triathlon swimming is different from competitive swimming. Although the goals are the same–complete the distance as fast as possible–triathlon requires you to be as efficient as you can in order save energy for the bike and run later on. In fact, many triathletes feel that the race does not really start until they get on the bike, and they think of the swim as a warm-up for the hard parts still to come.
Most of your triathlon swim training should focus on perfecting your technique and getting used to the distance you need to swim in the race. As your technique improves and you gain experience in open water, you can begin to focus on increasing your speed. Your greatest improvements in speed will occur as your technique improves. The more efficient your swimming is, the less energy you need to complete the distance in a faster time. But the biggest challenge you need to overcome is learning to use the resistance created by the water to your advantage, rather than letting it slow you down.