To be frank, there is nothing worth risking the use of that bell. While it's mildly interesting from a novelty point while on scuba, there really isn't that much to see. And if something goes bad you are in an overhead environment which reduces your already low chances of survival in case of blackout to pretty much nil.
Devil's Den is an interesting freedive just becuase of the catfish. As you can probably tell from the pictures, the cavern has a very small opening in comparison to the cavern itself. Greenery often obscures it even more. On an overcast day it can be VERY difficult to determine where the surface of the water is located.
In addition, every time I went there it was fairly dark. You will need at least 2 lights (primary and backup) and PLEASE check your batteries before you go. Tech diving safety measures apply to freedivers just like scoobies.
The pool itself is relatively shallow around the top, but is fun to play in. The bottom is silt covered rocks. Because inexperienced scuba divers often come here and bounce off the bottom repeatedly, visibility can be good or complete crap just depending on the day. The problem really lies in the overhead environment of a cavern. Anything deeper than 20-30 feet you will be swimming through openings between fallen slabs of rock or in completely submerged caverns. If you bump your head, you are screwed because any buddy breathing on the surface is gonna have a hard time finding you and you aren't going to float to the surface.
I used to live in that area, and while I went a couple of times it's just not as good a dive as the state park in Orange City. It's dark, often times hard to see, and once you have scouted the main cavern and played feeding the catfish there just isnt that much to do. I think I hit a max of 60feet in this cavern but it scared the bejeesus outta me. It was dark, cramped, and I found myself squeezing through small openings to peek at the scoobies. Finally I came to the realization that this was REALLY stupid diving (huge risk to see.....rocks and catfish in an overhead environment).
Orange City is the best freediving in the area that I have found. Fun, clear, safe, relatively deep, and manatees. The only other place that I found any depth and fun was a small sinkhole that was just north of Tampa in the waterways of a housing development. Although you sometimes have to compete with fishermen, in the winter the warm water provides a wonderful haven for the manatees and they are instinctively curious and LOVE to be scratched. While it's pretty murky on top, it clears up about 25 feet down although you will still need a light. Lot's of large fish to peek at and it's a fun dive. A word of warning though, coming face to face with a 1 ton manatee coming at you aggressively out of the murk becuase it wants you to keep scratching can be a......wetsuit cleaning experience. :t
Best of Luck and be Safe!