In March, "Seal" returned to the Haiti–Cuba area for exercises with Destroyer Division 4 (DesDiv 4). In April, she proceeded to New London, Connecticut, for overhaul which included modification of her main engines. In June, the submarine again sailed south, transited the Panama Canal, and continued on to San Diego, California, and Pearl Harbor. In Hawaii from July to September, she took soundings for the Hydrographic Office and participated in various local exercises. At the end of the latter month, she returned to San Diego, her home port into 1941.