I have sailed Alaska 9 times and 3 trips had very rough seas, so it is a myth that the inside passage is smooth. This also is not really a trip for a 2 year old, there are far fewer kids on board Alaska cruises compared to Caribbean. You may want to also consider a ship with alt, dining as a 2 year old will not be able to sit still though extended formal dining in the dining room and would be disruptive to other passengers. RCI has ships with enclosed pools but a lot of cruise ship pools are outside. All insider ports are interesting and very different.