Since there's so much technique involved in paddling, if you have trouble doing it or feel it all in your arms, you're probably paddling incorrectly. So try to take a professional lesson and master proper technique first.
Once you're paddling properly, the best way to gain paddling power is to improve your torso stability. This means training your hips and core to remain strong and stable as you paddle.
In the gym, the cable machines are a great tool for working on torso stabilization, with pushing, pulling, chopping and lifting movements, like this one. You can also try bridges (front, lateral, and glute bridges).
Another factor to consider is your shoulder health. Condition your shoulders, including your rotator cuff muscles, with exercises like Ys, Ts, Ws and Ls.
-- Craig Friedman