Swimming is great aerobic exercise that is also good for toning because even the parts of your body that aren’t actively moving are supporting you against the resistance of the water. Pool workouts are also unique because they provide firm resistance without impact.
Swimming Plan for Beginners - I've tried couch-to-5k, which I like, but this definitely starts out more intense than the running program does. So if you're literally starting from the "couch"... you might have to alter it to fit your beginning endurance level.
Summer means lots and lots of water. And I love switching it up from the gym to the pool to swim some laps. While I was once on a swim team, the amount of laps we would do consecutively was very hard for me and wasnt very fun at all.