There are several motels in Marianna - can't remember the name of the place I stayed in last, but it was ok and not too expensive - on the main drag near the west end of town. To get into the mill pond, you must check in with the Sheriff, pay a fee, and show them your cave or cavern C-card. Sheriff's office is on the west end of town. (Mill pond is northeast of town)
-----------
In addition to the springs in the Tallahassee area (there are several - Wakulla is most famous but getting permission to dive there can be a pain - I haven't been there so don't know details - you might check in with the WKPP folks ), a dive that's worth the effort near Tallahassee is out of Port St. Joe - the Empire Mica. She was torpedoed by the Germans in WWII, and she's starting to collapse - seems the whole ship is suddenly corroding to the point of weakness - so if you want to see her before she's just a pile of rubble you need to do so within the next few years.
Rick