Your occupation must be listed as Merchant Mariner in order for the Coast Guard and Homeland Security to communicate.
Call the TWIC help desk at 866-347-8942, select option 5. Tell the operator you want to verify or update your occupation. They will take your information, issue you a ticket number, then pass you on to a Tier 2 desk. It took about 10 minutes.
After 5 months dealing with the NMC, I was informed today that they had no TWIC information on me. I had submitted a copy with my application, and my evaluator told me a month ago he had my TWIC information.
In my case, my information was correct, and it seems to be a communication error between USCG and TWIC. But I would recommend everyone checking to make sure, if just to save a headache down the road. I know of one other person who used a Z-Card as ID when applying, but in the process, the TWIC office dropped the Merchant Mariner designation.