We've sailed both Caribbean and Alaska. They are NOT the same cruise experience. It is chilly on board, especially with the wind from sailing when out on deck. You are outside A LOT because you want to see the countryside and wildlife. Once you enter the passage, there is land on both sides of the ship throughout most of the cruise. The pool was open but rarely was anyone in it. We did use the hot tub the day we were in the Fjord after our excursion. The hot water was great but it was freezing outside. (snowing!). On shore, it was chilly in the mornings but by afternoon a short sleeve shirt was all you needed, it did get warm. I packed jeans, tshirts, and sweatshirts. I used them all. For the glacier excursion in the Fjord where it snowed, I had a Columbia winter coat with the zip out liner. I used both pieces. Gloves and scarf. I used a hoody & rain coat for the sea mist during the whale watching excursion. I don't think I wore shorts once.
The atmosphere is different too. There is still sail away.....but it is much more lax. Not the party crowd or atmosphere of Caribbean cruises. Shows were the same, comedians, piano bar, etc same.