The west coast can be a little chilly in December. The Arctic current runs down the coastline. The seas are usually calm and it will be warmer in Ensenada. But, take a light jacket or, better, a wind breaker. You will want something, especially up on deck or on your balcony in the evening. And, if it does rain, the rain will be cold. So, disposable ponchos are a good item to have. I carry them on all cruises, since rain is so not going to stop me from going to see the sights on any cruise.
Hope you have a great time. Ask as many questions as you want to.