I'm not 100% in agreement with this. Rowing machines are one of the lowest impact forms of exercise possible, and if used properly you should not have any problems.
There are two things to bear in mind:
. If you have never learnt to row properly prior to using a rowing machine, try to get some lessons on the correct technique. The people I see using rowing machines in gyms generally do not have good technique, and this could put additional stresses on the body. The first place to look would be here: http://concept2.co.uk/training/technique
In the bar on the left hand side of this page are links to various other information about technique, including a video. It is vital to get the technique right before starting to do long sessions/high intensity workouts. Asking the guy at the gym for a 30 second explanation of how to use the rowing machine is probably not much use.
2) If you are ONLY using a rowing machine as your exercise, it is probably worth adding two additional types of exercise into your workout programme:
. The main one for rowing is your hamstrings - if you have tight hamstrings then you will not be able to rock the body over correctly after the finish. This results in the back curving instead, which means that you are putting more pressure on the back than you should.
b) Core stability
. In rowing, the power is being transmitted from the "oar" (the handle of the rowing machine) to the "boat" (which is effectively where your feet are attached to the machine). In order to transmit the power effectively, you need to be able to hold your core (stomach and back) reasonably solid. There are many, many pages of info on core stability on the web, with various exercises that can be done with or without an exercise ball. For example: http://sportsmedicine.about.com/od/a.../a/NewCore.htm
If you have any questions about any of this then let me know and I will try to help!