Tom,
Ok, I guess I was reading Kevins Thread title: "Is the Belle of Louisville the oldest operating sternwheel steamboat"...

Regarding model bow tugboats with steam-powered screws, I'll add one to the list: HERCULES, 1907. Still operational in San Franscicso harbor:
http://www.nps.gov/safr/local/herc.html

They also have a sidewheel steam ferry EUREKA there, built 1890, still operational:
http://www.nps.gov/safr/local/eureka.html