Thanks for the replies. I plan to use a float switch to keep it topped off at 4 ft minimum excluding shelves.

I am hitting ground water at 4' but hope to dig a little deeper in one corner, maybe to six feet. My fear there is, how much water can I put my pond liner on top of before it just floats in place. Perhaps I could weigh it down with some cinder blocks and other structures.

I believe the ground water is 70 degrees. The 2' shelf answer is really helpful. In this part of Florida, there is mostly sugar sand and it drains quickly. Any overflow, I hope to direct to a drip irrigation for my fruit trees.

Do I need to cover the entire floor with gravel base? $$$ or is it okay to just use it in patches and on shelves?

Thanks again for taking time to help me think thru this BEFORE I do something that will be hard to undo.