Problem with outdoor gfi to switch to outdoor pond pump
Check for a neutral-to-ground short somewhere downstream

The behavior you're describing (a GFCI resetting fine, but tripping instantly when any load is applied) is symptomatic of a neutral-to-ground fault in the wiring on the LOAD terminals. I'd unhook and nut off the wires currently attached to LOAD, and then see if the GFCI resets and powers a load at this point.

If it does, then you'll probably need to replace your buried cable run; note that it's no good to use UF for a pond pump branch circuit, as its ground wire is bare, which violates NEC 682.31(A):


  (A) Type. Equipment grounding conductors shall be insulated copper conductors sized in accordance with 250.122 but
  not smaller than 12 AWG.


As a result, I'd redo this run using PVC conduit with individual THHN/THWN insulated wires (black, white, green) inside it; this fixes the Code issue with your existing install, in any case, for that matter.