Paddle. Next to kayak itself, paddle is your other big choice. And there is lot of paddles to choose between. Blade shape is probably the first thing you will look at. For touring, there are used paddles with straight blades and with curved blades (wing-paddles). Straight blade is the most usual and recommended for average paddler. Wing-blade is more professional and getting used with that may take some time, because it requires a bit different paddling technique. Another question is material. Usually, touring-paddles can be made up of aluminum/plastic, glass-fiber or carbon. Aluminum pipe and plastic blades are the cheapest combination. But weight is the disadvantage of such paddle. Fiber (composite) paddles (glass-fiber, carbon, kevlar) are generally more durable and with lighter weight. It is important to have paddle in right length. The simplest way to find it out is: stand straight, hold the paddle vertically in front of you (one blade on the ground), lift one hand up and if you can reach the edge of the blade with your fingers, paddle is in right length. Such measuring is suitable for normal-size paddler. Tip: Rubber water-stoppers for your touring-paddle are very reasonable idea as well. Also water-proof paddling-gloves for cold conditions.
Spraydeck is also a necessity for serious sea-paddler. Spraydeck doesn't let the water into your cockpit. Most of spraydecks are made up of nylon or neoprene. Neoprene spraydeck keeps warm (important in cold conditions) and it is more water- proof. But if you don't intend to make very long trips or paddle in very cold conditions, nylon spraydeck is enough. Tip: Also transport- spraydeck (without hole) is very useful.
Floating west (PFD). In rough conditions, floating-west can save your life. So buy one, which suits you the best and wear it always! There are great wests, which are designed especially for paddling. Such one doesn't disturb paddling, but - we repeat - can save your life.
Waterproof bag(s). All serious touring-kayaks have storage rooms, separated with waterproof walls and covered with waterproof hatches. Despite that, some water will get in those rooms sometimes. Because of that, it is good to put your luggage into special bags.
Navigation device. Murphy's law - "If you don't care, where you are, you can't be lost!"- is true of course. But usually we care about it and want to know, where we are. Depending on the nature, GPS or compass + map may be very helpful.