Seafood is always best if you are near the sea...eating ceiviche from a street vendor in Mexico City hours from the nearest seaside...what do you expect?
But no reason to be overly paranoid. If it were that bad, they would be running out of people in Mexico.
I believe 95% of Montezuma's Revenge has nothing to do with contamination. All water and uncooked food has bacteria on and in it but the selection varies by location. Travelers simply aren't accustomed to the local variants. Mexico just got a bad reputation because far more travelers go south rather than north.
PS: Reminds of the Christmas we spent in Merida. The grocery store (large, modern and clean) just put out a table and started throwing big chunks of unwrapped meat on it. I think it was beef but couldn't say for certain. It cooked up and tasted fine...and we survived.