Kayaking for Fitness
Kayaking is one of the best sports for fitness purposes and is becoming a popular fitness tool. It complements other fitness activities (like running), because it doesn't involve pounding and uses your upper body. The sport started getting recognition as a fitness activity because adventure races usually involved a kayaking leg. And so, more and more people began training in kayaks. It didn't take long for people to realize that kayaking was a great alternative and complement to other fitness activities, such as biking or jogging.
One of the great things about kayaking as a fitness activity is that you can get a great workout without inflicting the same kind of pounding on your body as say, running. Which means it's also a great option for people who have problems with their knees or hips. Being one of the few outdoor activities that really focuses on the upper body, it also offers people who like to exercise regularly a good balance to their routine. It helps avoid many of the common over-use injuries.
Boats for Fitness
For fitness paddling, any boat will do the trick, although a narrower kayak will glide through the water more efficiently and allow you to go faster. Of course, this doesn't really make much difference unless you're racing. And there is a real cost for the added speed, because as the kayak gets narrower, it becomes less stable. Olympic sprint paddlers use kayaks as narrow as 18 inches at their widest point, which is about half the width of some recreational kayaks. Just being balanced in these kayaks is a challenge for most.
If you're interested in paddling for fitness, ask around at your local outdoor store, paddling shop or paddling club. See if there are any programs of that sort in your area. Some clubs even run races for fun on a weekly basis, and these laid back events provide an excellent opportunity for getting to know other paddlers with similar interests.