I picked up swimming about 20 years ago as I wanted to compete in a triathlon and swimming was something I never did growing up. It is probably one of the most challenging exercises as you have to control your breathing but that is one of the benefits. When performing cardio workouts, you will be a lot stronger if you can control how you are breathing, especially when trying to get your heart rate back down.
Swimming laps will help develop your shoulders, chest, back and triceps, depending on what stroke you are doing. It is one of the few workouts you can do that will give you both cardio and strength work, as the water offers resistance for the muscles. Also, if you have joint problems or weight issues that hinder what you can do, pool work is an amazing alternative!