Hola from Mazatlan!
I live and have my busness in Mazatlan and have worked with many cruise ship passengers. I suggest you check out cruisecritic.com or cruiseopinion.com under the Carnaval ship Elation to see what they do as they come all year long.
There are many great things to do in Mazatlan but I'm not real sure about the other ports and these boards will be a great help.
I also agree with the passengers that buying on the ship is the most expensive way to take a tour.
If you have any interest in riding horses, check out mazinfo.com/horsebackride that's me.!
Either way have a great cruise and if you come off the ship between 8:30am and 9:00am look for me. I can hook you up with a good guide for what ever tour you want to take.
More info on Mazatlan check out mazinfo.com or maztravel.com they have some good coupons and great info.