Water has more resistance than air and also removes the effects of gravity. Running and biking are great cardiovascular activities, but your joints also take a pounding over time. Swimming you can do into your 80’s and 90’s.
What makes you healthy and fit is getting oxygen to your tissues and working muscles. Your heart is a muscle, and keep in mind that if you have a layoff it takes 6-8 weeks for muscle adaptation changes. Take it easy those first days back.
I like swimming because my whole body gets a workout. If you do good open turns your legs get stronger with every push off. Your core strength improves as your body rolls to breathe. Your kick gets stronger if you do more than a 2-beat cross-over and go to a 4-beat or 6-beat doing some sprints.
If you reach out further to get one shoulder more directly in front of the other before you make your catch, you stretch your lats and increase your shoulder flexibility and range of motion. Now you effectively increase the surface area involved with propulsion to make you stronger.
This places a load on your working muscles and your heart gets in shape to supply the oxygenated blood. Under pressure from hard repeats you open up more capillaries to supply your tissues and remove lactic acid.
Your lungs get a workout, too. Your intercostal muscles between your ribs and your diaphragm work to create a vacuum so you can exhale in inhale large volumes of air with each breath.