For truly cold weather insurance I also carry a flannel vest or ski jacket. Because rain is common in Alaska in the summer months, some sort of rainwear is also essential. I take along my golf rain suit, but a waterproof windbreaker and an umbrella are just as effective. But when we've flown up to, and landed on, glaciers, we were delighted we'd brought along warm socks, gloves, polartec fleece or parkas. We've found evening dress on the ship during our Alaska cruises to be generally more casual.
Formal nights can still be quite formal, but on most ships, all other nights are casual attire.
Therefore don't feel compelled to pack sports jackets and ties or extra cocktail dresses. If it turns cool, my next layer of clothing is readily available, and when the sun comes out, I have a handy place to store the clothes I remove. Even in the Caribbean, backpacks are handy for carrying beach towels, coolers and items you purchase along the way. So it's not something you'll only use in Alaska, if you choose to purchase one before you go. They're comfortable for walking around town, and combined with a good pair of socks, can keep your feet warm and dry in inclement weather.