The inside passage is like being on a lake all week. You can encounter choppy seas if in the Gulf Of Alaska. Not usually, but it happens. A well-traveled friend of mine (and former travel writer) once told me the worst seas he ever encountered was on a cruise from Anchorage to Vancouver. But it isn't something I'd spent any time worrying about.
As for the Sea Princess...don't let a blend of reviews sway you. As we've recently discussed in other threads, negative reviews often involve people with an agenda to settle because of some little issue that engulfed them. There is a lot to say about cruising from a port you can drive to and not have to fly.