I've been reading two books by Fodor's: "Cancun, Cozumel, Yucatan Peninsula 2007" and "The Complete Guide to Caribbean Cruises." The first book has site maps for each ruin. The second book has a ports-of-call section.
You don't have to buy the books, but they're nice to have on your cruise when your ports may change. Easy access to information instead of the onboard internet expense. I understand you can also read this same information on their website simply named fodors.com.
You've probably already researched the carnival.com website cruise excursions. The explanations say how long it takes to get to them and how long you get to look at the ruins. It may help to narrow down which ruin is best accessible from which port.
Of course, asking the experienced cruisers
here at cruise-chat for their recommendations is an excellent way to narrow down your choices. So I'll be reading this thread along with you because I'm curious what they have to say. Tulum/Xel-ha and Chichen Itza are ruins I'm interested in, but I'm still open for suggestions and wise direction.