On the three that we have done, most days were sunny and mild, BUT the wind, due to the speed of the ship and not having land to divert the air, made it feel cool a lot of the time. I frequently wore a couple of layers, with long sleeves, if I was out on the open decks.
I might also mention that in the Med, on land, we have experienced everything from short-sleeves days (but not shorts) to days in which I wore layers of shirt, fleece, and rain jacket.
But I am envious; it is my favorite kind of cruise!