"Fishing For Spies"
Major Thorpe (writer and artist), Mort Leav (pencils), Al Bryant (inks)
Published in Jumbo Comics #34, Dec 1941 - 8 pages
In the fishing resort of Guaymas in the Gulf of California, ZX-5 is undercover as a charter boat captain while checking out a tip that fifth columnists are at work there. His boat is hired by an American actress, her producer, her co-star, and an former German actor, some of whom are not what they seem.
Note: this is the first time in the series where a mention of Japanese activity is found.
Here are the pages I have of it.