When I read the topic, I was going to suggest Alaska (Inside Passage). Since we were so close to land for most of the cruise, it was very smooth. We went at the beginning of August. When was your Alaskan cruise? Was it the Inside Passage? I hear it can be rough outside.
With the ocean, I don't think you can ever be sure how smooth or rough it will be. On a recent Caribbean cruise, the sea would be like glass and then a late afternoon squall would change everything. It didn't usually last long though.