I have done cruises to the Mexican Riviera and would venture that being on a cruise ship schedule, you almost have to look for trouble to find it. You are in well patrolled tourist spots where the local economy doesn't need any bad publicity... word travels fast when bad stuff starts happening to cruise passengers and the ships have been known to change itineries away from places that aren't percieved as safe. There is plenty of corruption in Mexico, no doubt, but plenty of legitimate "golden geese" of tourist $, tax $ etc that are protected carefully in all those ports.
With ship security what it is, you will be photographed preboarding and you will have you ID card scanned everytime you board or leave the ship. If you fail to return on time, the cruise line knows about it immediately.
I have made wallet sized color copies of my passport page for me and all my family, and I carry those when I go ashore. I leave my original in the room safe.
FWIW, being surrounded by inebriated, out of the (normal) mind cruisers, their are many easier targets for bad guys to focus on that the typical reader of a forum like this one! It a little like not having to be as fast as the lion thats chasing you, only faster than the guy next to you.
I carry as little as possible when I go ashore, split out different credit cards from different banks between my wife and myself, one carries the ATM card, with the back up on the ship.
It also does not hurt to call your CC companies and you ATM bank that you will be in Mexico during the time, so they don't freeze your card.
I find the tourist areas of Mexico to be very welcoming and freindly.