Two very common mistakes made in swimming are to assume that a
) your core i.e., abs, lower back aren't as important as your arm and leg strength b
) new swimmers have a tendency to "break" the knees while swimming and kick with the knee down.
When you watch experienced swimmers do endless laps, notice how you barely see a ripple from behind them, while new swimmers will create waves and whitewash with their kicks.
Focus on kicking from your hips. Here's an excercise that you can do right now to visualize this.
Move away from your desk while sitting in your chair, lift both legs up, DO NOT break your knees, move your legs up and down, place your fingertips on the tips of your thighs that connect to your hips..feel them flexing while you lift your legs up and down? That's the muscle that will initiate the kick and what experienced swimmers use; rather then kicking with the lower portion of the legs.
Also, during your workout, grab a kickboard, and flip on your back. begin to kick while holding the kickboard above your chest
(only your elbow/hands should be visible above water, not the entire arm
). This strengthens the abs and is a great way to work on your kicking as well.
In triathlon swimming, the kick is not as important as your arm pull, however, if you can minimize a bad kick, you minimize lost expended energy that could be applied toward other things.