I have done this trip twice and both times found the Caribbean to be calmer than the Pacific side. We went all the way to Vancouver and from San Francisco on it was so cold that every jacket in the store sold out - that was also in April. It was quite rough going up the coast from Mexico on but not enough to cause sea-sickness, just hard to walk around.
We are doing this trip again in April. I'm sorry that I can't remember when the formal nights are but they were definitely sea days.
Like you, I like to know all the details - the more you know the smoother the trip IMO.