For December, you'll probably want to take along a windbreaker and some long pants. With luck you won't need them but sometimes a Norther drops down and it can get a bit chilly down by the water. This effect also depends on where you are from and how long you are staying on the island.
Often at the beginning of my trip, while the locals are all wearing their woollies, I'm still quite warm enough, but within a week or two, I'm wearing my woollies too.