Following an extended shakedown cruise in the Caribbean Sea and a post-shakedown yard period, "Seal" departed New England in late November and proceeded to the Panama Canal Zone to commence operations out of her home port, Coco Solo. Arriving on 3 December, she conducted local operations off Balboa, Panama, and off Coco Solo into January 1939, then proceeded to Haiti where she participated in type exercises prior to Fleet Problem XX. That exercise, to test the fleet's ability to control the approaches to Central America and South America, was conducted during late February in the Lesser Antilles.