Hi, Maxi - impact forces are not the only issue to consider when thinking about joint stress. Because swimming is an overwhelmingly upper body activity, the shoulder joint especially can become overstressed especially if one does not vary the stroke. This is particularly true if one does the front crawl and only turns to one side to breathe (which is often the case) - the opposing side that is performing the downstroke has to do a lot more work. You can offset this some by varying your stroke among crawl, backstroke, breaststroke, butterfly and sidestroke, but IMHO only to a certain degree.
Also, IMHO traditional swimming isn't one of the best cardio exercises out there, primarily because again it is an overwhelmingly upper body activity (and the upper body muscles are much smaller than the lower body muscles albeit more numerous) and because you are performing it in a horizontal position planing the surface of the water. The whole point of swimming skill is stroke efficiency - i.e. getting the most distance out of each stroke with the least amount of effort.
I often suggest to people that if they want to keep traditional swimming in their program as an integral part of their cardio training, to frequently alternate their traditional swim laps with kickboard drills that force the legs to work alone without any help from the upper body. Flutter and dolphin kicks are good for these.
a-jock