Google Earth is absolutly accurate.
There are programs to cache Google Earth inages, and on a PC the images are cached for a while anyway.
Some locations in the South Pacific are pitifully charted. Tonga is a good example where some of the islands in the popular Va'vu group are a half mile out. That would be fine... But hell for seeing the reefs in between!
I think Google Earth is excellent for cruisers, not only for navigation but for the simplest sounding things that tun out to be quite important... Finding, even, the dinghy dock in a new area! Just zoom in and look for all those little grey blobs.
Yes if the pics are good you can tell a lot looking at a combination of chart and the aerial view. There's a tremendous amount of very useful info you can see in the aerial pics.
I set the Cache to max 2GB ( In Google Tools, options, cache) then delete the cache and browse the all the coastlines and save the cache file for each destination.
Offline ( aboard) I then load the saved cache into the cache directory for the area I want to look at.
Using XP the cache is stored here:
C:\Documents and Settings\Xis\Local Settings\Application Data\Google\GoogleEarth but you will need to enable "show hidden files, folders, and drives" in Folder Options before you can see these folders.
For the PC challenged you could get someone to set up a simple s c r i p t file or even drag the appropriately named location folder contents into the cache folder.