[hamsterpower]

I agree that water should definately not share a conduit with electric ever!
The depth of the trench usually is due to the local frost line. 3+ feet here in MA, but I don't know in OR. There is cable availible that can be buried legally without conduit, but that too could conflict with local code. Keep the cable in conduit all the way down the wall and up to the outlet mounted to the tree at the end. It can be without conduit for the horizontal run if deep enough. You should add a foot of sand on top of the cable.
There should not be any problem adding this new recepticle to the existing two garage outlets. Those in the garage *should* already be GFI protected. If they are not, add your GFI to the first in the chain. Do not however put two GFI recepticles on the same circuit as they tend to cancel eachother out.