When men were assigned to an Armed Guard Center (AGC), the ships they served on do not appear on the service record, because the ships they were generally assigned to were either merchant ships or troop transports, and not "Fleet Ships". One way to find this information is to request a copy of his personnel records. You can do this on-line at
Click the link for "Request Military Records" on the right side of this page. There is no charge for this, but sometimes it takes many weeks (or months) to get the information. If you are lucky, there will be something in here that points to the ships.
Ron Carlson, the web master for the Armed Guard site also has some other sources that seem to work well. He usually chimes in on these sorts of requests (sorry to volunteer you, Ron).