The Ichnetucknee exists because Florida is basically a massive chunk of porous limestone covered with sand. Water soaks in and flows through voids in the limestone. In many places, the water works it's way to the surface and forms springs. This type of porous limestone terrain is called "Karst" topography, after a similar region in Europe.
The voids beneath Florida range from tiny cracks to immense caves. The famous Florida sinkholes that make the news are usually the result of a weakened cave collapsing and causing the surface depression known as a sinkhole. These sinkholes are so common in North Florida that there are large companies that do nothing but sinkhole mitigation.
Okay, that's your geology lesson, let's see some more pictures of the float trip.