Take what you would expect to wear in spring and early summer and include some rainwear and sweaters, you can always peel off a layer but if you haven't taken it you can't add it. ... Neil.
The simple answer is that no one can say for sure what the weather will be like.
However, have a look at http://www.wunderground.com/history/ and you can have some indication what the weather might be like.
So far as clothing are concerned the best advice is to take clothing such as light shirts etc and jumpers that can be put on top during the cooler evenings. I think the term usually used is take layers!!!
What you will find is that ships are usually warm except of course, if you go on the balocny or deck at night.
Not even Michael Fish can predict the weather so why do you think that we can?!?!Simple answer is that God alone knows......literally. And he isn't on line at the mo.
We did the same cruise over xmas and the new year and i can tell you it was hot every day every port we stopped at xmas day it was 84degrees and superb same on new years day .