the happiest of exercises

Once you start swimming regularly, it won't take you long to go from flabby to fit.

Ever see a flabby dolphin or a weak-looking competitive swimmer? We didn't think so. That's because swimming is a great way to increase muscular strength and muscle tone -- especially compared to several other aerobic exercises.

Take running, for example. When a jogger takes few laps around the track, that jogger is only moving his or her body through air. A swimmer, on the other hand, is propelling himself through water -- a substance about twelve times as dense as air [source: Yeager]. That means that every kick and every arm stroke becomes a resistance exercise -- and it's well known that resistance exercises are the best

Lower Stress, Higher Spirits and a Better Brain

William Wilson wrote in the 1883 book, "The Swimming Instructor": "The experienced swimmer, when in the water, may be classed among the happiest of mortals in the happiest of moods, and in the most complete enjoyment of the happiest of exercises."

Wilson probably didn't know this in the 19th century, but all that happiness was likely due to the release of feel-good chemicals known as endorphins -- one of swimming's most pleasant side effects. In addition to a natural high, swimming can also evoke the relaxation response the same way yoga works on the body. This is due in large part to the constant stretching and relaxing of your muscles combined with deep rhythmic breathing. Swimming is also a meditative exercise, with the sound of your own breathing and the splash of the water acting as a mantra of sorts that can help you "drown out" all other distractions.

https://health.howstuffworks.com/wellness/aging/retirement/10-health-benefits-of-swimming10.htm