The sternwheel Str. BENJAMIM GUIMARAES was built in 1913 and still operates in South America. There are 3 photos of it on Don Sander's web site here:

It's the boat that was mentioned in a posting a few months ago as having a recessed sternwheel, but it's more like a wheel with a boilerdeck overhang.

I don't know of any opertional sternwheeler older than it... The BELLE is so close, it's definitely #2, at least.