Swimming is traditionally the most difficult of the three triathlon disciplines to master. Most people opt to swim with the freestyle stroke (or overhand crawl) for this part of the event. However, proper performance of the freestyle stroke is very contingent on correct form and technique. Knowing what the ideal form is along with regular swimming drills with an awareness of the ideal form and technique can be the best way to improve overall performance in this field.
Here's a great instructional video that shows correct form