I'm glad to hear you're doing the ship tour. If Gil and I had it to do over, I'm sure we'd both pick a ships tour this time around. To refresh, (I believe it was Gil's third or fourth post:
http://www.luxurycruisetalk.com/cgi-bin/ubbcgi/ultimatebb.cgi?ubb=get_topic;f=1;t=003882

Having spent six months of my college years in Mexico, I learned through first hand experience, what to eat and what not to eat. Having been in Santo Tomas, I'd say better safe than sorry. Like Gatowoman said, nothing that can't be peeled. (I wound up sick on and off for four months because of some strawberries from a very exclusive restaurant in Mexico City that were supposedly safe.) Stay away from the salads, etc. No ice cubes and only bottled water or better yet, have a beer.