There are two keys to packing for Alaska. The first is layers, bring lots of layered clothing so you can add and remove as needed. At night on the decks or balcony you'll need extra layers.
Second, be sure you have a wind and water repellent outer layer with a hood. Like a wind breaker but get on about 4 sizes too big so you can put it on over a heavy coat/parka, etc. if needed. The Pacific N.W. can be fickle and a daily shower or constanty mist or drizzle is not uncommon. Staying dry is the key to staying comfortable. The winds and temperatures at night can be uncomfortable at best if your wardrobe is not appropriate. May and September being the start and end of the season, it's cloder then than the rest of the season. The temps may be low 40's but it feels like 20's when the ship is cruising and the wind blowing.
Have a great time!