There is, I think, a fairly common misconception that rowing is all about arms. When I started rowing I got a lot of comments about how jacked my arms would be. And while my arms for sure are getting stronger, everything else is too. As one of the coaches said recently, rowing is both aerobic and anaerobic: it's a full body workout, requires a lot of leg and core strength, gets your heart pumping and all your muscles sore. And if you're not sitting up tall, you're going to have problems. So while rowing is definitely helping my core, I also need to work on it outside the boat.
I asked said same coach for a list of any core exercises that would help, and she sent me this article from Real Simple with six core moves. It's got some nice variations on standard planks and crunches.
In particular, I like how this rope climb approximates the angle we lean back at when in the boat. I always really feel it starting in my lower abs, and it forces me to think about my posture, as does this move.
Do you have any favorite core strengthening moves? I'm always in the market to switch it up!