Is it dangerous?

As with any water sport, there is always inherent danger. However, if you follow some basic safety precautions and stay within your skill limits, you can control most of the risks.  

The great thing about whitewater kayaking is that you can decide how much risk to take on.  If you just want to be outdoors in an incredible environment, you can paddle easier rivers where the risk is minimal.  If you're more interested in the excitement of whitewater, you may want to increase your skills until you're comfortable in class III or IV rapids.