Are there any sources of spatial information about river rapids worldwide (with difficulty category)?
American Whitewater offers the National River Database.
The river list selection page gives an overview of real-time information that we can track on a state-by-state basis. We track information for the United States, Canada, Mexico, the Dominican Republic and Costa Rica.
Data is mostly from gauge and user reports. An example of a gauge page looks like this
I don't see an export option, but if you contact them and have good motives, then maybe they can help. Otherwise, the HTML is easy enough to "parse".
As for the "difficulty", the state pages have a table with the Class.
And individual rivers have more info
Including some geo-info