I really liked that cruise. Here is a thorough review of the Mercury on the itinerary that you are going on.
Also, you can find a ton of information about the ports of call at http://www.MexicoPortReviews.com
With the exception of Manzanillo, you can do pretty well just going on your own depending on what you like to do. It is a great cruise. Let me know if you have any specific questions.