I have cruised out of NYC at least 10 times. No two trips the same. The first day has ranged from glass to 40' foot seas. No storm off the coast no heavy seas. I think April should be a low probability of heavy seas.
One thing about the Gem and all the Jewel class ships they are very good in heavy seas. Normally you really feel nothing until the seas get over 12'.
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.
Gotta love it. We were on the Majesty during Hurricane Bonnie. 40 foot seas. Not to be believed. Everyone was sick except me and an iron worker from South Boston. We were drinking beer at a bar that was open.