I tubed down the Ichetucknee too when I was younger. I remember the water being crystal clear and ice-cold.
There was a hole at the end that pumped water up from an underground stream. As the guide was explaining to the group that it was impossible to touch the bottom without diving gear, my father came up from the bottom with a handful of sand. The guide freaked out. My dad was in ridiculous shape though, and used to lobster dive without tanks as a hobby.