Now then, when I say cardio I am referring to the three classic forms of cardio; running, cycling, swimming. There are others, such as interval training and plyometrics that are more modern approaches to cardio, but these will be covered in another post.
Swimming can have many more benefits than both running and cycling. First off, it could save your life. If you don't know how to swim, stop reading this and get some swimming lessons NOW, it could quite potentially save your own or someone else's life.
Next is a quite well known point, but it is still valid. Swimming is much better for your joints, this means that you can enjoy going for a swim, long after you are too old to go for a run. This also means that it is a great method for physiotherapy and generally avoiding injury if you have weaker joints.
But now we'll get into the main reason why swimming is much better than cycling and running, it uses your whole body. This means that you burn more calories, as you are using more muscle groups at once. It also leads to a more proportional gain in muscle mass (think of how good a swimmer's body looks) as opposed to running, which doesn't really lead to any significant muscle gain, and cycling, which leads to massive legs but an underdeveloped torso. It also helps your overall body co-ordination, as you are synchronising your legs, with your arms, with your breathing.
Finally, swimming can greatly improve your respiratory system; due to the fact that you have to control your breathing, alongside with the increased resistance to expanding your chest in water as opposed to air. This leads to bigger, more efficient lungs that will aid you in all other exercises. In other forms of exercise, such as running, some people (such as myself) struggle to breath properly and get out of breath quickly or become wheezy (due to asthma or other conditions). Personally, I have found that swimming allows me to exercise for much longer without getting out of breath whilst in the pool, but has also allowed me to stop my breathing from going out of control whilst on a run. Although I have heard of other people having their breathing worsen due to swimming, so be careful if you have any respiratory conditions.
In short, swimming could save your life, and leads to a more rounded, whole body workout that can increase your cardiovascular fitness greatly, whilst going easy on the joints.
Alternatives to swimming that also give whole body workouts include; interval training, rowing, plyometrics (jump training), as well as yoga, aerobics and many more that burn a lot of calories, and help to build muscle.
It would be best to try all of these different methods, to see which one suits you best. Find the one that you enjoy the most, as this means you are more likely to stay motivated to keep at it for a long time, but also switch it up so that it doesn't get old.
As always, Good Luck.