How cool does it get on the Hawaii cruises at nite? In other words, if I want to take a walk on the deck, will I need a jacket or are shorts and a short-sleeved shirt enough?

Are you starting your cruise in Hawaii or somewhere else in the US.

I´ve heard, that the first two and the last two days on the Pacific can be very cold. On one cruise all sweatshirts were sold out the first day.
I would recommend a sweatshirt and a light jacket also for the islands. Hilo has different weather than Kona, it can be much colder and some rain.
You can also have some rain on Kauai, when you are making a shore excursion.
I still have time to think, what I will pack, but in about 90 days my luggage must be complete for Hawaii.

Have a wonderful cruise.
Marita

Almost all NCL ship's itineraries start and end their cruises in Honolulu or Maui. There are just a few sailings from California and Vancouver when ships reposition from Alaska or drydock.

The weather in Hawaii at sea level doesn't get to cold in the evenings, the coolest temperatures are experienced just before sunrise.

Here's the average temperatures for Honolulu by month: http://www.world66.com/northamerica/unitedstates/hawaii/honolulu/lib/climate

The lowest average monthly low is 16.35C (61.43F) and the highest average monthly high is 28.95C (84.11F).

Do you need a sweater or light jacket for 61F?

We were never cold, even though it rained some days