higher voltages are run all the time underground....how does it get from a transformer to you meter? Sometimes overhead...sometimes underground.  I've run (GFI protected of course) several 115VAC distributions to the front and back yard, all in conduit.  You can use direct burial cable but still risk having it cut into, conduit is just more difficult to accidently put a shovel thru it.  Having the distribution off a GFI (in 5 yrs never had it trip) gives the secondary protection if there ever is a problem. I even have remote controlled relays functioning as well to turn on and off the waterfall. 
 
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