I'm looking for information I can visualize in Google Earth concerning rivers, such as rapids, put-in points, take-out points, campsites, etc. Any idea how I can look for this kind of information?
In Canada you find this type of data in the CanVec datasets provided by the Natural Resources Canada Geogratis portal. There is a 1:50K (metres) nation-wide coverage and it is the same data used for the Canada 1:50K topographic maps.
There is also a new generation of topographic maps being produced also by NRCan. Coverage is still still limited but you can find if your area of interest is available here.
These maps/data include some of the features you are looking for but for more detailed information you might have to search the databases at provincial/state level.
If you are looking for USA data, than the NHD dataset has rapids information (note, not all the states seem to be collecting this feature type).
This is specialized stuff, so you're best off inquiring among paddlesport organizations. In the US, begin with the American Canoe Association, which has a link to "water trails" information. It also provides links to local clubs, which traditionally will have the best and most up to date information about rivers near them--don't expect there to be a detailed national database that is current or of high quality. Some clubs may have databases of geospatial information by now, but of course the availability varies from club to club. If these obvious sources don't pan out, the Philadelphia Canoe Club maintains a useful set of links to Web sites worthy of further research.