re: For being topographically challenged...
The Frio river is like that. It is so clear, you can see your shadow 12 feet down. It looks like you are floating in air.
You just don't see clear water like that anywhere else along the US Gulf Coast. It has to do with Florida having all that limestone. The rest of the US Gulf Coast has silty sediments. If I remember my geology correctly, Florida’s limestone was a reef deposit when the area was under a shallow tropical sea.