You may be surprised by how much warmer it is. Sitka never goes below freezing, even in winter. Much of the Alaskan coastline is a temperate rain forest. The ocean currents make a huge difference, so be prepared for warmer, not colder. That's why I suggest layers...you can always shed a layer or two. I've done Alaska in May and Sept, and both times had days when a t-shirt was adequate. You just can't count on that.
NCL no longer has true formal nights, it is optional. A lot of people do dress up, but at least you will still be able to eat anywhere you choose without worrying about the dress code.