I have 2 areas like yours, but I have about 6 months in the dry. I have done the same brainstorming as you. Lately, I have been think of sinking 2 7/8 drill stem into concrete about one every 8 feet. Then welding drill stem on top, elevated above the waterline. Kinda-like the top rail of a pipe fence. And then I would have a piece of drill stem, like a rib every 8 feet. One on each side. The only thing I can’t figure out is what I will use to put between the two rails that I can drive on. Heavy gauge sheet metal would be too expensive (I expect). Maybe I could weld drill stem side by side lengthwise. My expanse is only about 50 feet on one road and 6 feet on the other. Below are pics of each.