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'.
Day 1 Feb 2007
Day 1 Feb 2006
Day 1 Feb 2005