The temps onboard cruise ships stay at a fairly constant temperature day and night. You can adjust it in your stateroom, but throughout the ship its nice and cool. I do pack a sweater to bring to the dining room in the evening just in case its really chilly.
for some reason the dining rooms are always cooler than the rest of the ship. The ships are very comfortable to walk around in at night. The show rooms can get warm, same as the bars and casinos. I always have a sweater or wrap with me at night.