A squat focuses on the strength in the legs that is needed for the kick in swimming. From the hip flexors to the ankles, squats are an all-in-one leg workout.
Standing up straight with your feet at shoulder length stance, go into a squat position, bending at your knees, keeping your head up and chest forward. Push that butt back. Try using a chair to give guidance when lowering the body down. When lowering try to barely touch the seat and then straighten back up.