From what I understand your GPS should be good to about 10 meters or better.
Most GPSs somewhere in their menu should be able to display "current calculated error".
As has been mentioned before, there are biggere variances in the sort of mapping we use than the GPS.
As stated soo many places and times, there is no substitute for a visual look out.
This is why beacons are beacons.
Now consider what "Google Earth" is it is a free to use aerial photograpy product.
It will be automaticlay stitched together from a large number of frames, many of them not taken from directly above....the image of my house is clealy taken from the south.
If google earth covers the entire globe, what datum does it use and what grid system does it use to resolve the irregularities of the earths shape......
I think you are expecting far too much from google earth.
If you want to get mapping that is accurate in the ground (or water), you fist must have a datum, then you have to have a corrected portion of a grid, that grid then has to be referenced to some on the ground real references, then the maps have to be drawn accuratey in relation to all of that.
From my limited experience, even the official published maps for navigation have limited accuracy.
It is important to understand the concept of mapping.......it is a picture designed to show you approximately what should be there so you can confirm it by ither means and thus establish you location.
cheers
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