Maurine - As has been said earlier - Stick with the ship's tours and don't sweat it!!!
For one thing - you won't be in port in the later hours of the evening and you won't be walking down darkened alleys. The cruiselines do a great job of trying to keep their passengers away from the less than desireable areas!!
Foreign travel is really quite safe and the tours will see to that, Tiajuana is a border town with all the peculiar problems of a border town, where you will be in Mexico is a much different environment!! (and much safer!)
If it helps- I'm planning to do the Mexican Riviera in early 2012- and I am anxiously awaiting the cruises schedules for the late 2011 to early 2012 seasons!
BillyBuzzy