I have same outdoor GFCI box and X10 light dimmers feeding my lakeside dock and boat ramp lighting. This is so I can turn the lights on / off from an incoming boat past dusk if they are not on already.
I even hit them with a pressure washer to keep the bugs off them and they still keep ticking.
I have this circuit:
underground wire (UF grade)
Grey covered box
GFCI (this is important for outdoor!)
Back into plastic conduit
Once in a while the GFCI has tripped due to water intrusion, but a quick reset and back in business. Better that than someone getting zapped!