I am on the Star Dec 8 - Ports are Acapulco 11am til 12:30 am, Zihuatanejo/Ixtapa 7am til 2pm, Puerto Vallarta 9am til 5pm, Cabo 8am til 4pm. Hope that helps - I am interested in snorkeling and shopping also.
Cabo will probably be your best bet for snorkeling as f-mattox says. Very clear warm waters. In January you might even be able to see the whales as you head out since it is the birthing season there in Dec. and January. Have a great trip.