Rowing and cycling have always complemented each other in my life! I believe they are excellent cross-training for each other. In Ireland, we've had several women being on the national squads for both sports!
Because you're in a sitting position for both, working-out heart-rates tend to be more similar than in running or swimming. And in both, the load worked by the leg is determined by hard you can drive the muscles, rather than the impact of your body weight in running. And they are both conducive to getting obsessed with the sport!
Differences: I think cycling favours the lighter woman, and in rowing, the taller and stronger the better. (Huge generalisation - I know!) Also, rowing is much more technically demanding. Starting out in a single scull is an excellent beginning - you have to have good technique, or you risk toppling yourself into the drink!
Have fun!