You need to find out what the code is in you area - I cant imagine getting deep enough with a pressure washer to satisfy the "depth rule" of outdoor wire that is not in PVC. I think it is 12 -18 inches. (note: I hope you used outdooor rated wire !!)

With PCV you can go much less. I want to say 6 inches, but I'm not sure.

I also agree with the other posters - I think the whole outdoor run should be GFI protected. But a GFI outlet offers just as much protection (to devices "downstream" as a GFI breaker).