Tone Stomach - Swimming Exercises

I had read once that Arnold Schwarzenegger in his younger days utilized swimming as one of his main cardio and muscle toning exercises. He used to swim everyday and incorporated swimming in as much as 50% of his workouts. Arnold Schwarzenegger will go down in history as one of the most symmetrically balanced body builders ever. In other words all his muscle groups were in proportion to his body size. He obviously knew what he was doing.