I found that it varies. Our first cruise was May 2002, and I thought it was extremely cold in the WDW theater (probably hovered around 68-70). In fact, I tracked down a housekeeping cart to grab a bunch of towels to wrap around me and the kids because we were freezing! The dining rooms were a bit better, but there was still a cool breeze, particularly if I was wearing a sundress (probably 70-72 in there). I had taken pretty white sweaters for my DD and I, and we used them at dinner almost every night. DS and DH were fine, but they didn't have bare shoulders.
Our next cruise was Jan 03, and the overall temps were much warmer (I'd say most places were 75 or so). Now on that cruise, the outside temperature probably hovered around the mid-70s during the day, so that may have had something to do with it. First cruise was more typical tropical weather, high 80s during the day.
If it were me, I'd just bring a dressy sweater for the ladies, and maybe a light jacket for a boy child (men usually don't have bare arms inside).