"Shawmut" was reassigned to the US Air Department after refitting at Boston Navy Yard to serve as an seaplane tender. The ship carried out the majority of her duties tending the amphibious aircraft off the eastern seaboard of the United States from Key West, Florida, to Narragansett Bay, Rhode Island. Her voyages frequently took her to foreign ports including: Guantánamo Bay, Cuba; Kingston, Jamaica; Samaná Bay, Dominican Republic; and Trujillo, Panama. On 17 July 1920, she was reclassified as CM-4 (Cruiser-Minelayer #4).