You are correct. In sandy soil, the pond water level will quickly equalize to the water table.

I like TJ's idea above. If you have lots of room you could even "bench" the pond, where you dig a large overall basin of the size you want and make the top bench the dimensions you like that are just above the water level for your wildlife pond, and the second bench is the bottom of your pond.

(However, if your soil is really sandy, you will not be able to dig very deep below the top of the water table. Wet sand is NOT cohesive, and will just spread into the excavator bucket hole you just dug in the bottom of the pond.)

Another option would be to use an artificial pond liner. They are fairly inexpensive these days, and in the smaller sizes you and some helpers can still easily manipulate the liner. You could excavate down to just below the water table and the sand would then spread to give you a nice smooth bottom. You can then use your spoils from the pond hole to anchor the edges of your liner. Using a liner and your hydrant, you could design the pond level at any elevation you desire, and easily keep your pond full.

There are many threads on pond liners in the archives, or you can search some basic articles on the internet.

Good luck on your new wildlife pond!

I have a groundwater wildlife pond on our farm that I had to deepen two years ago to get it below our "drought" water table. It got very heavy use when the two creeks in the area completely dried up.