You can swim in water and walk on land. If you are a triathlete, you add cycling to the land mix. Duathletes miss out on the swim fun. If your body allows you to run, this is a great way to cross train, adding some extra variety, training gains, and scenery to your workouts. You can still add some walking to your training mix.
Swimming has some good things for runners, too.
No one works hard, swimming lap after lap, just to get injured. But that may be the cause of swimmer's shoulder. Lap after lap - done the wrong way too often or too fast. The year 2000 brought a few new stars in 50 meter pools. It also shined up some the old ones and saw a few begin to lose their flash.