Swimming is often regarded as one of the most effective forms of exercise. While true, the swimming that most people do equates to the cardio equivalent of floating in a wading pool. Simply put, swimming is an unnatural act that differs greatly from walking, running or even cycling.
Until someone gains a considerable amount of experience in the pool, it is impossible to treat swimming as another form of cardio. Competitive swimmers do not have this problem because they have found their natural stroke and that natural feel for the water that allows them to vary speeds, vary stroke rates, and use swimming as true form of cardio. Unfortunately, the majority of people using swimming as a form of exercise do not have a natural stroke or a natural feel for the water.