This may surprise you, but rowers weren't born with a perfect six pack of abs and Incredible Hulk-like shoulders. They happen to be in a sport that revolves around how fast they can move their arms from the front to the back with great water resistance.
For this reason, professional rowers have crazily built backs and pectorals. If you enjoy the build of these rowers, you can actually get it yourself without having to join a rowing team. Check out this video to learn how to do a one arm dumbbell row to tone your lateral muscles.
Make sure to use a bench to really activate both your back and your core. By doing only 1 arm at a time you are able to recruit more back fibers and nerves.
Make sure to focus on your "Mind Muscle" connection.
Also make sure as you watch the video to punch with your opposite elbow to fully engage your core.