One more WatiN gotcha to post – in a freshly created project with WatiN added (via nuget), creating the new instance of the IE object will (always? often?) result in an exception thrown with the error message that the Interop.SHDocVw dll missing. The apparent solution for this is-
-Click on this dll in the References section
-In the properties for this reference, find the property “Embed Interop Types”, and switch it from True to False.
Try running again and it should work now.