My group did this itinerary last Oct and had a great time. Weather was good, seas were calm.
My suggestion for Manzanillo is to take a ships tour, not much else there to do and the taxi drivers don't speak english. In Zihuatanejo we took a tour by hiring a van right at the beach area where you get off the tender, we also had him drop us off at Carlos N Charlies in Ixtapa where we hung out on the outside patio bar on the beach. There is a swiming pool, dance floor and restrooms all right there for your use as long as you order drinks, which by the way were great and cheap. Another great thing to do is a massage right there on the beach. There are canopys set up in the sand near the waters edge, so relaxing and only $10.
Acapulco we had Roberto the port authority guy set up a driver for us so we didn't have to wade through all the others at the pier. Our driver was great and spoke perfect english.
E me if you need any other info.
You will enjoy!!!