First off, paddling a boat on the ocean doesn’t make it a sea kayak. A sea kayak is determined by what a boat is capable of, regardless of the location it’s paddled. Sea kayaks are designed to handle rough waters, to give the paddler control of the boat, and to cover distance efficiently. In addition to design, most modern sea kayaks will share certain safety features that are important when paddling anywhere away from shore. So even if you just paddle on a calm lake, a sea kayak may be the perfect boat for you.
In general, sea kayaks will be 16′ to 17′ long (some more specialized kayaks will be a little longer or shorter). They are normally 22″-24″ wide (again, with some outliers). Most of them are sit inside kayaks with relatively small cockpit openings designed to handle a sprayskirt (though some Sit-On-Tops are designed to handle rough water). They should have deck lines around the perimeter and toggles at both ends that allow a person to hold onto the boat if they are in the water. They should either have built in flotation chambers or the ability to add flotation of your own (so they won’t completely fill with water in case of a flip). This is what makes them safer in rough water – as long as the paddler knows what they are doing. The other design characteristics are the same for all boats, but we’ll look at what they mean for sea kayaks in particular.
Rocker is one of the most important characteristics in any boat. It’s how banana shaped the boat is. A lot of rocker makes a boat easier to turn – more responsive. This can be very useful in rough conditions when the waves are pushing a boat around and you need to control it. Boats with less rocker will have a longer waterline and track better (go straight). That makes them more efficient (faster) and makes it easier to cover distance.
The hull shape is another key component that greatly influences the performance of a boat. Rounded hulls are the most efficient – that’s what racing boats will have – but by themselves are rather unstable. Flat hulls are most stable (in flat water) but are slower and get tossed about more in rough water. So most boats are a blend or compromise of these features. Rounded hulls will still have some flat area in the middle, flat hulls are often angle a bit to the middle, and a common shape is a ‘shallow V’ that adds a bit of a center keel line to help with stability and tracking.
Along with the shape of the bottom, the shape of the corner is another key point. How the boat transitions from the bottom to the side affects how stable the boat is when leaned on its edge (very important in rough water). Soft chines, with a gradual corner, generally have a wide range of stability that depends on the roundness of the overall design. Hard chines (sharp corners) often have a very stable point after which the boat just flips over. Multi-chines attempt to combine the best of both worlds.
Finally, most sea kayaks (though not all), have either a rudder or a skeg. The main purpose of both is to help the boat go straight when the wind is trying to turn it. Rudders have the added advantage of being able to steer the boat as well (steering is normally done with your feet). Skegs are simple fins that stick into the water to help the boat track; they can be raised or lowered but they do not turn. Boats with a lot of rocker will have a skeg, since they are easier to turn on their own. Boats with minimal rocker will come with a rudder, since they can use the extra help to turn.
Once again, there is a choice of materials in sea kayaks. Composite boats using fiberglass or kevlar and carbon are very common. They help save weight while providing stiffness and performance. Plastic kayaks are less expensive but heavier. They also require less repair and hold up to abuse very well. Your type of paddling and proximity to rocks is the main factor for what materials you will want to go with.
So regardless of whether you paddle on the ocean or a lake, a river or a lagoon. If you want fast and efficient, if you want to be able to carry gear, if you want control in rough water, then a sea kayak is the perfect boat for the job. But there are many styles and picking the right blend of characteristics will give you a boat that does what you want while allowing some versatility.