Paddle Company, ProPel recently unveiled a line of kayak paddles designed to be paddled by somebody with one mobile arm.
Here is the description from the website: The paddle system is comprised of a grip and adjustable cuff. The cuff and grip are also adjustable along the paddle shaft to adapt to various arm lengths. The paddle can be used with either the right or left hand by easily repositioning the cuff or grip to the appropriate side of the shaft.
The website has a little video demonstrating this really promising product. While you are there, watch the demo video of their prototype canoe paddle.