I attached an image of the wire. Its black/red/white with a ground wire, there is also another green wire wrapped around it on the outside. Right now I mainly would like to install a receptacle beside the fountain to power it and other things if need be. This area is between the house and the shed. I was not planning to run power to the shed just yet because of the work involved, but I have already installed a sub panel.
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This wire should be sufficient for all your needs. There may be a local requirement regarding the wire. Possibly UF is not permitted over 20 amps, certainly it must be buried 18 or 24", so that may color your decisions.
If you want to keep it simple for now: Pull out the two pole 50 amp breaker, save it for later. Replace it with a single pole 20 amp breaker. A GFI breaker would not go amiss. Decide to use either the red or black for the circuit, and put a nut on the other one. Don't cut it unless there is plenty of it in the panel.
Bring this cable up along some solid support to a weather proof box. The box to be a bit on the large side. A 4x4 weatherproof box with 3/4" openings will permit you to put a large cable clamp on that cable. You can extend the 6 ga with short lengths of 12 ga, and wire them to an outlet.
If you used a conventional outlet at the panel, use a GFI outlet.
Either way, use an "in use" cover on the box.
If there is a wall that you can mount on this on, good.
Use a piece of 2x6 pressure treated secured to the wall to carry the cable up the wall to the box, mount the box on the PT, and secure the cable near the bottom and near the box.
There may be a height requirement for the outlet. 24" should do it.
You could do all this on the cheap and wait to extend this cable to the new panel in the shed later.
I could also imagine an inspector insisting that the UF not be exposed above ground. In which case, some 3/4" plastic conduit (actually, that stuff may not fit into 3/4", you may be forced to go to 1") screwed into the bottom of the box and extending below the dirt by say 12" (or if the cable really IS buried deep, go all the way to it) would be good practice. Little to no chance that a stray tool could cut your cable.
But all that is much less of a worry if you have a gfi breaker.