August 3rd is National Watermelon Day, which recognizes the refreshing summertime treat enjoyed at picnics and fairs. With that, enjoy this image of the ersatz Great War-era patrol boat USS Uncas (SP-689) showing bluejackets munching said melon amidships, circa summer 1917.
Collection of Robert S. Waters. Donated by Mrs. Alice W. Thomas, 1972. U.S. Naval History and Heritage Command Photograph. Catalog #: NH 98352
The gun appears to be a rather elderly circa 1880s 3-pounder, which is what you would expect for a 60-foot wooden-hulled powerboat taken up from civilian service.