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It depends on the soil type. With really tight clay soil, you can fill it with a pump and it will stay full. Well.... sort of - you would probably have to top it off on occasion. In sandy soil, the pond water level will be the same as the ground water table level.
Since you know that the ground water drops by 20 ft in summer (that's a huge drop) I suspect you have sandy pourous soils in your area. Your agriculture extension service will have access to soil maps in your area.
If you are considering constructing a mud pond, you may need to install a liner and place a layer of soil on top of the liner. The liner will cost at least 30-60 cents per square foot. There are also techniques for mixing bentonite into the soil to make it less pourous, but it seldom works very well.
In constructing the pond, you push soil from the center of the (future) pond to build surrounding berms. You may only have to excavate a foot or so to have enough soil for building the berms with enough left over to place a layer of soil back on top of the liner. With a high winter water table, you would want to keep some water in the pond through the winter to avid having the ground water lifting (floating) the liner.
A pond constructed in this way will be much easier to manage than a water table pond. You will be able to completely drain it via gravity and/or with a pump. Complete drainage is important for harvesting the fish and, equally important, for drying the sludge accumulations, getting rid of undesirable organisms, etc.
Water table ponds are notoriously difficult/impossible to properly manage. You cannot drain them. They will be too deep in the winter and too shallow (or dry) in the summer. A 25 foot deep pond would be really difficult to aerate.
-stevehopkins
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