Is kayaking dangerous? The short answer is: NO. Now the caveat - there are dangerous situations out there in the world of kayaking (as there are crossing the street) but like crossing the street, we can control these situations with a high degree of confidence, given the right tools and the right knowledge. Since the most perilous types of kayaking are by sea and whitewater, I'll restrict my answer to concentrate on these pastimes. First, I would say beyond a shadow of a doubt that you should - heck, MUST wear a helmet if you're anywhere near a shoreline.
This is especially true of whitewater kayaking safety. It only takes 13 pounds per square inch of impact to permanently damage your head. This is certainly not said to discourage or scare you! On the contrary, you should wear a helmet so you can remove this worry from your mind and have a good time. There's nothing macho about being a vegetable, even if you think you look dorky with head protection.
With regard to kayaking safety, buoyancy is key for staying safe and stable. With that said, I'd also recommend a personal flotation device - this sometimes means wearing a lifejacket although they're often referred to as PFD's. You should keep the PFD on and zipped up at all times - this alone could save your life. Specifically to sea kayaking safety is the question of how far is safe enough to venture from shore. When you're out there, think to yourself, "Can I safely swim back to shore from here?". If the answer is no, get to paddling until your a safe distance from land.
Sometimes, you will become capsized which means you will be turned over in your kayak completely upside-down. Now, before ever going it alone, I would consider it a must to learn how to do an eskimo roll in order to right the craft and get stable again. If you can't do this - learn now from an experienced friend or instructor. The eskimo roll is a pretty counterintuitive exercise that you can't just learn by force of will.
But the alternative to learning how to roll is to abandon your kayak and exit the craft. Once this is done you can swim back to shore and think about how to retrieve your very expensive kayak. Given the choice, I'd take some lessons! ;-) Another preventable hazard is the weather. Check the conditions before you go out on the water and use common sense. Assess your own skill level and don't be afraid to cancel an outing should it become necessary.
And if I can impart just one more thing. Until you become an advanced sea or whitewater kayaker, ALWAYS go out in groups of two or more. It only makes sense. Kayaking is such an amazing rush sometimes but it doesn't always have to be an extreme sport. It has aspects that should appeal to all ages and lifestyles.
But in order to have the time of your life, you'll need to remove doubt from the equation and be as safe as possible by using common sense and kayaking safety.
Kayakadvice.com offers lessons for the beginning paddler as well as a buyer's guide and kayak vacation planner for the kayaking enthusiast.