Kayaking is that rare upper-body-centric cardio exercise. One of the benefits of aerobic work is that it increases the number of capillaries in the muscles worked; paddling shares this perk with your chest, back, shoulders, and arms, and the natural resistance of the water means that you can't just coast along and expect to remain in motion.
Your lats and shoulders are in constant motion, and your legs, while not doing a lot of weight-bearing movement, play a role in stabilizing you.
Amp It Up
Felix Apfaltrer, president of the New York Kayak Polo Club, suggests sprinting out 25 meters on still water in a light, maneuverable kayak, turning, and then sprinting back to where you began. Rest 30 seconds, then repeat the sprint. Do five complete up-and-back laps, adding 10 seconds to each of the rest periods as you go.