The best weather for the Mexican Riviera is December thru mid- April - it is the mildest. Puerto Vallarta is in the tropics. As November moves to December, the temps start to cool down. That being said, weather is unpredictable everywhere. Of all the times we have been to Puerto Vallarta (whether or not on a cruise), the hottest and most humid day we ever recalled was in mid-December - not the norm. In Cabo, many times it has been cool with a mist or light rain in the mornings and warm to hot in the afternoon in the winter months.
We always bring light jackets for cooler nights on the ship, especially on deck where it can also get very windy. The A/C has never been too cold for us, but might be for others. Also, it could be cool/cold/rainy in Long Beach and warm in Mexico.