I agree with Shoreguy. We're cruised out of NYC probably many times and the seas were smooth as glass and then we had cruises where we had 18-22 ft seas for 48 hrs straight. That's one thing you can't predict - just like the weather.
You can usually expect rough seas for about 6 hours when going by North Carolina. But, during that time, look for circular rainbows. I've seen them on 3 different trips.
Have fun and don't worry about a thing.