swimming is great for everything.. mornings and evenings are best if your doing public pools because most pools have lanes up and no kids splashing around.
I don't think you should use it to replace any muscle workouts, a little bit of free weights is good for a lot of things, it also works as a good cool down for weightlifting. However, it is all you would ever need for cardio.
If you are a novice swimmer buy this book: Total Immersion Swimming by terry laughlin. It teaches you how to swim efficiently and has a lot of good drills in there to help keep you in shape, there are some good youtube videos that this guy did as well that can help you with the visuals. I have read a handful of books on the subject but this one alone turned me from a 50m swimmer into a 3km swimmer within a few weeks.
Waterproof headphones and a waterproof ipod case can make swimming laps enjoyable and even more stress eliminating too. Highly recommended form of exercise, one of my favourites and I have done them all
