It's great for your lungs

When your face is under water, oxygen is at a premium. In turn, your body adapts to use oxygen more efficiently, says Walton. Plus, it learns to take in more fresh air with every breath, and expel more carbon dioxide with every exhalation. A study in the Indian Journal of Physiology and Pharmacology even found that swimmers had better tidal volume (the amount of air that moves in and out of the lungs during relaxed breathing) compared to runners, one of the under-the-radar benefits of swimming. This results in lower resting heart rates, lower blood pressure, and, as you'll see next, better running performance. source