The modern whitewater kayak is made of plastic for its ability to absorb impacts and its resistance to abrasions. Under the deck just forward of the cockpit will be some kind of support device. This is usually a partial bulkhead made of mini-cell foam. This is a closed-cell foam that aids in flotation as well as support of the deck, should the kayak become pinned sideways against a rock or other obstruction.
The bottom of the kayak should be either flat or have only a slight curve side to side. This is to aid in turning and to get the kayak on plane, in the case of the flat bottom. Kayaks made for whitewater tend to be shorter with a lot of rocker, or curvature from front to back along the bottom of the boat. This rocker is what makes the whitewater kayak so maneuverable and quick turning. This does hinder speed and requires a lot of correcting strokes when paddling flat-water stretches. The payoff is that the whitewater kayaker can pick her way through or play in the rapids.
Curving up quickly, the sides' shape gives the first-time paddler the feeling that the kayak is tippy. Most whitewater kayaks have excellent secondary stability with little primary stability. Primary stability is the feeling that the boat is stable when you first get in. However, once the boat reaches a certain point, it will quickly capsize. The ability to resist this is called secondary stability. The shape of the sides helps when rolling the kayak back upright after capsizing and helps the kayak to be put on edge for faster maneuvering and play in rapids.
Decks of whitewater kayaks should be free of rigging and hatches. Rigging can create a safety problem if it gets hung up on something in or above the water, such as tree branches. Hatches can leak, especially with the force of whitewater. Leaks are not just annoyances--they allow water into the kayak, which can upset the boat's stability.
Cockpit size and shape needs to allow the use of a skirt while being large enough to allow a quick exit, especially in case of capsize when hanging upside down under water. This is called a wet exit and is an essential skill.
Outfitting is done so the kayak fits the paddler. Kayaks are worn, not sat in. Most whitewater kayaks have good outfitting from the factory. Things to look for are a seat that holds the paddler in place and thigh, hip and knee braces positioned for comfort and to make the fit of the kayak snug. Better whitewater kayaks will have an adjustable bulkhead as a foot rest, and not foot pegs. This is a safety feature to reduce the chance of injury should the kayak hit a solid object head-on at higher speeds.