Both are designed to be built using wood strips. What’s the difference between a surf ski and a sea kayak. Beyond having an open cockpit, not much. Here’s a great explanation from one of Björn’s recent posts:
Sea racer or surfski?
What is the real difference between the Sea Racer and the Spindrift? Or in general – what separates surfskis from sea kayaks or multisport kayaks?
Well, it turns out the only sure sign is the cockpit configuration. In everything else there are lots of crossover crafts floating around, defying categorization: sea kayaks with surfski maneuverability and surfski with the calm movements of sea kayaks and all kinds of hull configurations.
Surfskis were from the start developed to run off the wind on large swell reaching extreme speeds, and to surf the wave fronts in control. To keep the position and direction on those waves, extreme maneuverability was needed. To control the craft, precise steering and a large rudder was necessary. The stem was high to avoid diving in the gigantic waves of the native waters in Hawaii, South Africa, California, Australia – while the stern often became little more than a long tail with a rudder. The fore part was deep and narrow moving the lateral center forward to balance the large rudder, while the aft part was wide and shallow to prevent the stern from submerging in speeds close to or above hull speed. Surfskis became elite tools for tough oceans races. But eventually the market evolved to include also crafts more suited to “normal” paddlers with touring ambitions.
Sea kayaks on the other hand evolved towards strong tracking, rather than maneuverability. The designer sacrificed some maneuverability for comfort on long passages – in automotive terms they have the soft somewhat imprecise steering with a healthy margin for errors of a family car, rather than the precise and attention-demanding direct steering of a race car. Of course some of the control is lost in the process. Have you tried to surf a steep wave diagonally in a sea kayak, you will know what I am talking about. But lately we have seen a lot of sea kayaks with almost surfski-like maneuverability and control.
The post has much more information on the design of the Spindrift and the Spray.