While I realize most of the people visiting this site will already have a  good understanding of what I'm saying here, I'll say it anyway,  just in case you're a newbie.

Face it - kayaks go in the water. And humans are not amphibious,  regardless of what our modern technologically-induced arrogance tells us. Part of the attachment people have to boats of any kind is that the vessel is the only thing between you and death, and your mastery of the vessel is your insurance. So, it is incumbent upon the boater to be sure that all reasonable precautions are taken. Most  "victims" in sea kayaking are victims of their own ignorance. Not all, but most. It sounds a little cold of me to say it, but it's true. You truly do have your life in your own hands. You may never have any kind of incident, and most people never do, but what if?

                                                                                                            -Whistle
                                                                                                            -Signal Mirror
                                                                                                            -Flares
                                                                                                            -VHF Radio*
                                                                                                            -Smoke Marker*
                                                                                                            -Streamer*
                                                                                                            -Air Horn
                                                                                                            -Strobe

You should always have at least a whistle and a mirror, no matter where you go. I carry flares whenever I go in salt water, and always have my radio when I'm alone or crossing shipping channels. My whistle and air horn are permanently attached to my PFD. Letting bigger vessels know you're there can prevent the need for the use of any other signal. Flares, smoke, strobes and streamers  should only be used when there is a clear and imminent threat to life and/or property (US Federal Law). Use the air horn and radio primarily to signal other craft. Whistles are best suited to getting the attention of other kayakers.

                                                                                                        -PFD !!!!!!!
                                                                                                        -Skills !!!!!!!!!
                                                                                                        -Immersion clothing (Wetsuit/drysuit when appropriate)
                                                                                                        -Paddle float
                                                                                                        -Spare paddle
                                                                                                        -Dry clothing
                                                                                                        -Reflective materials (On your PFD, and maybe your boat)
                                                                                                        -Boat floatation
                                                                                                        -Sprayskirt
                                                                                                        -Bilge pump
                                                                                                        -Towline (At least 25 feet of it, readily accessible)

Most deaths in sea kayaking are due to hypothermia. You have to dress for the water, not the air. Water conducts heat away from the human body many times faster than air. If you're wearing a drysuit, avoid cotton like the plague. Drysuits leak from time to time, and cotton does not stay warm when it gets wet. Wear polypropylene, fleece, wool, or silk under your drysuit, because the suit itself provides no thermal protection at all, it just keeps you relatively dry.  I prefer a combination of materials, with silk as the base, because it dries the fastest, and stays warm the best (In my opinion). Plus, it just feels nice ;-)

Drowning is usually preventable by wearing your PFD at all times. How simple is that?

Boat floatation is another no-brainer -- if it won't sink, you can almost always get back in. Bulkheads are good, but they do fail occasionally. Hatches leak, or get blown off from time to time. If you're going out in rough conditions, or really cold water, do yourself a favor, and use float bags fore and aft. And make sure your hatch covers are tethered to your boat (Hint to the British manufacturers - they make some really great boats, but I have yet to see a British boat with tethered hatches).

Which brings me to the second most important thing you can have -- skills (PFD being #1). Good bracing, sculling, and boat control skills are essential, as are routinely practiced rescue skills. The best one of all to know is rolling. I know a lot of people will tell you otherwise, but think of this: If you can roll, you already have advanced bracing, sculling, and boat control abilities. This will keep you from capsizing in the first place a lot of times. It's not hard to learn, unless you have some sort of physical impairment (And some people do), and is actually a lot of fun to do once you've learned it. It's also the fastest way out of a capsize situation. Don't be intimidated by it -- I know some fairly uncoordinated people who can roll a kayak like it's nothing. Mind over body is all there is to it.

I keep my paddle float handy as a last-ditch item. If I can't roll, re-enter and roll, or get an assisted rescue, THEN I use the paddle float. It's a valuable item, but I'd rather rely on skill before objects. No false sense of security this way. I also always have a spare paddle on my back deck. I've had to use one, and have learned their value the hard way. You never know when you're going to break  your paddle, or lose it in surf, etc.

                                                                                    -First Aid kit (A good one - don't be cheap about it!)
                                                                                    -Knife
                                                                                    -Multi-tool (Leatherman,etc)
                                                                                    -Duct Tape
                                                                                    -Waterproof matches
                                                                                    -Backpacking Stove (Cold weather)
                                                                                    -Extra water
                                                                                    -Flashlight with extra batteries

All of these, except for the water, should be in a drybag in your front or rear compartment at all times. First aid kits, knives and multi-tools speak for themselves. So should fire. Duct tape is another matter. It's not just a guy thing -- it's probably the single most versatile survival item in your kit. Duct tape can be used to:

                                                                                    -Patch a hole in your boat
                                                                                    -Splint a broken bone
                                                                                    -Hold a bandage in place in wet conditions
                                                                                    -Patch a drysuit/drybag/sprayskirt
                                                                                    -Build a shelter (Lashing material)
                                                                                    -Repair a broken paddle
                                                                                    -Secure a damaged PFD to your body
                                                                                    -Replace a lost hatch cover

But don't get the idea from this that sea kayaking is so incredibly dangerous - it's not if you know what you're doing, and take some sensible precautions. In fact, it's a lot of fun. The things I have listed here are not a complete list of stuff, by any means, but they should be your basics.

     Better sources of info than listening to me:

        The complete book of Sea Kayaking, by Derek Hutchinson
        The Coastal Kayaker's Manual, by Randel Washburne
        Nigel Foster's Sea Kayaking, by Nigel Foster

All 3 are excellent primers for getting started. There are a lot of other good books out there as well, but these are in my opinion the 3 best. You could also add Eskimo Rolling, by Derek Hutchinson, and The Bombproof Roll and Beyond, by  Paul Dutky once you get started.

But most of all, have fun and be safe!