Brian,
When you say "in the service" do you mean the Merchant Marine Service, Navy Service, WWII Army Transport Service? What years? After WWII? Was he married or did he marry while "serving" as a marriage certificate and also a child's birth certificate usually has groom's/father's occuption, where employed.

Have you seen this website?
http://www.usmm.org/

If your grandfather is still alive don't ask him to remeber specic things him just get him to start talking..then interject comments asking such things as...when did he first get the idea or desire to "go to sea", did he go to a "school" what was his job on board ship, how long was his first voyage, does he remember where he boarded his first ship, ports visited, shipmates names, Captain's name, what cargo, places took liberty. Where was he when FDR died? Did his ship dock in LA/Long Beach Harbors? Seattle? San Diego? Did he ever sail through the Panama Canal?

Good Luck