It is a myth that is is always smooth sailing inside passage. 3 out of 10 of my trips there encountered rough sailing, once lasting 3 days. ALL of those were round trip Vancouver and when they sailed more inside passage than they do now. If prone to seasickness TAKE PRECAUTIONS.
i have been on the water all my life ( i live by the mississippi ) but i was sorry i had forgoten to take my bonine when we left vancouver. even though the water was smooth i did get sea sick the first full day and had to lay down in the cabin. but after that i was fine. it is true what they say about taking the meds in the morning before you get on the ship and again the next morning. wish i had remembered.
We cruised from Seward to Vancouver in 2001. The Gulf of Alaska was quite rough - in fact several of the crew members were sick. I hear "Bonine" is the very best for sea sickness. The rest of the Inside Passage is normally very calm except for a very brief time 12 or more hours out of Vancouver. The Inside passage is just beautiful and is like being on a river for the most part.
We have taken many, many, many cruises(twice a year) in Alaskan waters since 1972 and weather and water conditions are difficult top predict. What we have found is if the vessel is within the Inland Water Passage, where you are relative close to the shore, the motion of the ship is smooth. Once in a while in the area of Ketchikan you might hit some rough water. In the Straits of Georgia near the Seymour Narrows there can be some rough water due to strong tidal flow.
Usually heading south after you leave Ketchikan, the ships head out to sea on their journey to Victoriia or Vancouver, there can be rough seas. In recent times some cruise lines, such as Princess transit a good amount of the time going out to the open seas and then head into a port. They then go out of the port to the sea and "pop" into the next port. Heading nortth, after you leave Skagway to Seward, you may have some rough seas.The same for leaving southward from Seward to the lower Inland Waterway cities such as Skagway, Sitka, Juneau, Ketcikan.