Penny, that's great! Welcome to the "Left Coast". I don't believe you've been on Sapphire Princess before (?)--I know you're going to love her as much as I do.
As susanmaz says, the Mexican Riviera is no "Grand Adventure"; but it is a very relaxing and pleasant getaway and it will be nice and warm in the three ports.
I know you've read that Mexico's West Coast waters are not as warm and clear as the Caribbean, and this is true--but they are still very pleasant; and Cabo San Lucas is your best bet for good snorkeling and all beach activities. Kayak tours get you great views of the famous Los Arcos rock formations.
If you want to really absorb some of the colorful Mexican culture there is a tour in Mazatlan that takes you into some of the villages in the foothills of the Sierra Madre. Or there is a walking tour of Old Mazatlan that gets you up-close to this charming old city. Or just stroll the Golden Zone, with its great shops and restaurants. And many just take the short water taxi ride to Stone Island for a very inexpensive day of beach fun and good food.
In Puerto Vallarta a day at one of the all-inclusive beach resorts is a good choice--there aren't any really good public beaches. Or just take the short cab ride to downtown and stroll around; the Cathedral, shops, restaurants and an artisans village by the river are all within easy walking distance. Or if you're more adventurous, there are zip lines, jungle canopy tours, ATV and bike tours and hiking. But even in December Puerto Vallarta is hot and humid.
So Penny, whatever you do I know you will have a great time. I'm really happy you are finally coming out our way.