If you want to build just once I would not use wood. Treated wood is only resistant to rot. Life span varies by each piece of wood. One piece might rot in a couple years, and another from the same batch will last 15 years. Search the web and you'll read many examples of a piece here and there rotting sooner than expected.
In a fence, a deck, or even a retaining wall some rot is not that big a deal. A 4x4 post can rot almost all the way through and the fence will stay up. A pond has constant stress. A small amount of rot can result in a total failure.
Here's a wood pond I built years ago. It was 1,700 gal, 11' x 7', 34" deep. 4x6 pressure treated beams, lapped corners, 1/2 plywood inside sitting on a concrete slab that was part of the driveway. Threaded 3/8" rod anchored into the slab and screwed tight at the corners and mid point in each wall. When I filled it with water it started to bow out. I had to flip the top 4x6 so the 6" side was horizontal...still bowed some. That was an 11' side, a bit less than 3' deep.
And what the sides look like (different pond, same type of structure).
You can put ties across the top to hold in the sides, but kind of ugly and certainly a hack.
My point is, I was right on the edge of failure. A little rot and I'd be hosed. I torn down the wood pond after a few years and there was some rot. This was in San Jose CA, not a lot of rain and no soil contact.
Mainly I built with wood because I was used to working with wood. I had little masonry experience. I assumed wood would be easier, faster, and cheaper. Wrong, wrong and wrong.
In my next pond I used concrete block. The trick was to dry stack. All that skill needed for mortar joints...not needed. Dry stack and fill the voids with concrete. It was easier, faster and cheaper. Way stronger and should last 100 years or more.
You do want a good foundation...but that's true for wood too. You can ask your local building department the minimum footer size for your local soils. 8" wide block doesn't require any footing if you soil density is high, but I don't think that's the case in Florida. So maybe a 12" wide by 6" thick with 2 lengths of #4 rebar. Because you have to dig out the pond there's not much extra digging.
You can add some vertical anchors into the footing, but in this case that's probably not needed.
If you make sure the footer is poured level you'll be able to just start stacking the block. I use bond beam block, not the standard type block. Bond beam has more room for concrete and easy to add horizontal rebar which is important. Some vertical rebar is good too, but no need for too much. Stack a couple of courses and fill with concrete. Repeat. Goes pretty fast. Use string to keep the walls straight, although this isn't a requirement, you can bend the wall if you like. Use a running bond pattern, but, because there's no mortar joints you'll probably end up with a one gap on each wall in each course. Just use bricks to fill the gap so the void between can be filled with concrete. Or you can make a small wood form to cover the gap and hold in the concrete.
I anchor a treated 2x4 into the top course. Easier to nail the liner to. Then cap with whatever material you like.
Let cure 4 weeks before filling with water.
