Thanks to Mr. Raghavan Srinivasan for his reply.
There is a small bug in file.cio generated by ArcSWAT 2.3.3.
I manually edit file.cio, 4th line from the top with the following...
6/16/2009 12:00:00 AM ARCGIS-SWAT interface
and it can finally be opened with VizSWAT.