You will love to be pampered with Crystal.She is bigger than Silversea so you will not feel the motion.
Never sure of the sea,sometimes it seem to be calm but suddenly the wind
comes and make a rapid change of the weather.Choose a season that is most favorable to your expectation like January,February and March.
Hope it helps a bit.
Nice cruise anyway.
I cruised with Royal Caribbean. As far as calm seas goes; last year my wife and I did the mediterraean. Italy-Croatia-Turkey-Greece-Italy. The Adriatic Sea was as calm as a Lake in summer. Weather in May was perfect. The larger ships have stablilizers which make the seas more enjoyable.
Freedom of the Seas Dec 11
Allure of the Seas: Jan 11
Voyager of the Seas: May 09
Splendour of the Seas: May 07
Explorer of the Seas: Jan 04
Majesty of the Seas Jan 99
You will feel motion on all ships if the seas are bad. Crystal ships are still relatively small compared to the giants appearing today. There is no totally smooth cruise; that is, it may be smooth one week and rough the next. I think your best bet would be an inland passage Alaska cruise that sails from Vancouver or Seattle. As you don't head all the way to Seward, you spend very little time in open waters.