Alva Leland Watkins, shown in the first image, is believed to have served on SC 304, which spent time along the Mexican coast. The Ship Movements book shows SC 304 leaving San Diego for La Paz on 1918-10-01, and leaving La Paz for San Diego on 1918-12-09.

Thanks to Gary McKee for submitting the photo scans to The Subchaser Archives.

Photo Set - click to view large versions
Alva Leland Watkins on SC 304
SC 304, Mexico
Submarine chaser SC 304