Efficient paddling doesn't require a great deal of upper body strength. Some conditions do require power - surfing, sprinting, and paddling against a strong wind. But most of the time on a kayak trip you'll be trying to keep the boat moving forward at the cruising pace of 2.5 to 3.5 knots. Beginning kayakers are all arms when they paddle. All they can think about is pulling the paddle through the water.
Beginners seem to be clawing at the water.
An accomplished paddler's stroke looks longer and smoother. Even when the beginner and pro paddle at the same stroke rate, the pro's kayak moves faster, while the pro seems to be hardly working. The paddling pro is working with a different model. Rather than thinking about pulling the paddle through the water, the pro thinks of the paddle as almost stationary and tries to pull the kayak past the paddle. The long kayak paddle is used as a lever to move the boat forward.