Safety
Kayaking alone is rarely a good idea, especially if you're braving strong waves or difficult currents. If you're going on your own, give somebody at the coast (like the person you rent the kayak from) a copy of your planned itinerary and estimated return time, so they can call for help if you don't show up on time. Beginners should not attempt strenuous or long routes, and instead stay near the coast or within eye distance of a lifeguard or boats. Many rental shops offer classes, sometimes for free, to teach you defensive kayaking and how to deal with the unexpected.
Paddle Specifics
Paddling in the ocean is not the same as paddling in calm river waters. For those who have never kayaked in the sea before, it may take some practice to get used to the waves and currents. A good way to handle this is to have a plan beforehand, created by learning the direction of the currents, estimating the best route and planning a departure and return time. This will help establish a plan of escape in case things become difficult due to an incoming storm or changes in the current. Also, it's important to check the weather report before heading out to sea, not only to see whether rain is coming, but also to establish the direction and intensity of the wind, which will significantly affect your paddling. Unless you're an expert, a forecast of storms and high winds should mean "cancel trip," as it is extremely difficult to handle high waves and unpredictable moves during a storm on the ocean.
Feathered paddles are better for ocean paddling, but they are more difficult for the beginner kayak enthusiast to use, as they don't provide the same resistance against strong currents. If you're a beginner, stick to standard square blades and instead learn about the paddling maneuvers that will help you navigate waves and currents.
Gear and Accessories
Aside from the proper kayak and paddles, there are a number of items that can make the trip easier and safer. The obvious ones are a life jacket and a helmet, especially if you're kayaking in a place with a high surf. Kayak emergency kits, sold at specialty shops, usually contain a collapsible bailing device, flares and a first-aid kit. A GPS and a VHF radio (to call for rescue) are also a good idea.