The location of the ground relative to the hot wire is really not important provided it has solid connection with moist earth. The ground can easily be places near the energizer and connected to the ground terminal. There is no reason that a hot wire needs to make a complete circuit back to the energizer and multple lines can be
led off from the energizer. The key is to prevent unwanted grounding by branches or other vegetation. From the sound of the rated fencing coverage, you should easily get the attention of the deer provided they come in contact and don't either scoot under or jump over the single wire. In really dry times, the resistance at the surface soil level can reduce the shock, but changing the ground position can't do much for this and placement near a
pond is perfect. No harm will come from the ground wire unless the circuit is completed!. Some critters may also push through if startled by the shock, especially if they can't see the wire.
The goal is have the target animal complete the circuit to provide a strong warning that this is a "no go situation". However, a single wire doesn't sound like the best option for protection but it is an understandable attempt to reduce cost and material. This minimalist approach may deserve experimentation provided there is nothing too precious that you are protecting. Sometimes it is best to have a layered or three dimensional fence to discourage intrusion. If a deer or in my case moose is approaching the fence it slows them down visually and if there are a few strands of energized wire on an angled 2 by 2 projecting outward at ear or nose level, it makes for an unpleasant surprise and even moose can learn to stay away from the fence. We use five feet of welded wire fencing below and keep it grounded so that climbers such a black bears are grounded between the fence and energized wires running above to reduce there visits. Our fences are pretty lightly built and a fast moving moose or brown bear could blast through them with little effort! It would take a lot of expense to make a fence that stopped this. Many approaches can work, and there are always unexpected ways for the systems to fail!
A lot of jittery electrons still seem like the most cost effective way to reduce unwanted critters from the garden. Good luck with your fencing.