Wear a Lifejacket
Wear a life jacket before you get into a kayak. Make sure that everyone in your party is wearing one; it's especially important for children to wear them. In many states, life jackets are required for young children and during certain months of the year. A life jacket will greatly improve your chances of survival if an accident happens. If you enjoy whitewater kayaking, wear a helmet as well.
Take equipment along to make yourself noticed, especially if you are kayaking in a channel with shipping. A whistle will allow you to alert nearby ships that you are there, and it will allow you to signal for help if you get into trouble. Bright colored clothes are another good idea, since they will make you more visible to large ships. If you are kayaking after dark, before dawn or after dusk, use a kayak deck light or other all-around light source to identify your position to others. A headlamp will help you to see into the darkness.
Don't Go Alone
The same thing is true for kayaking that is true for virtually all other wilderness activities; you are always safer with someone else. Bring at least one other paddler if at all possible. If you are going to be going on an extended trip, prepare a float plan detailing where you are going, who is going and when you are returning. Leave that plan with a friend; arrange to check in with him when you return. That way, if you don't come back within a reasonable time of when you are expected, he can contact the police and begin a search.
Dress to Get Wet
Even gentle, forgiving kayaks tip over sometimes, and the gentlest days can still have rogue waves. When that happens, you will get wet, and you will get cold. Dress for the unexpected. If the water is colder than 60 degrees Fahrenheit or the water and air temperature added together are less than 120 degrees Fahrenheit, you need to wear a wetsuit to avoid getting hypothermia.
Check the Weather
Check the weather forecast the night before your trip and again an hour before you go out. If there is a good chance of a storm, big surf or high winds, stay in. If you are kayaking in a coastal area, check the tides as well. Tide swings can create strong currents in coastal rivers and bays, which can suck inexperienced paddlers out to sea or drive them into shore.