Open water divers can dive in either the main spring head or Catfish Hotel. The main spring is a large bowl, about 25' deep, lots of fish, etc. Cafish hotel is a karst window/sink about 500' away from the main spring. It's covered in Duckweed, but goes to about 85' deep. No lights allowed for OW divers in Catfish. Purpose is to keep them from going into the cave system, which connects Catfish to the main spring and goes back upstream from there a long ways to several other sinks.
Manatee is a high flow cave, so there's a significant siphon in Catfish, another reason for OW divers to be careful and not get past the grim reaper signs into the cave system.
It's a decent dive and at $10 a good bargain. There are limits on the number of divers, but it's usually not crowded. Call thepark office to get info on the limits. One issue is that there's no fills anywhere close, so you'll have to bring multiple tanks if you want to make multiple dives.
Also, be prepared for the duckweed, it gets into everything. Make sure you purge your back-up reg once you are under the duckweed to blow out any that may get in your reg. You don't want to have an air share and end up getting a mouthfull of duckweed on your first breath!
I've dove the cave system once, both upstream and downstream from Catfish, but didn't spend much time in the OW section so I can't offer a lot of insight on that. I did snorkel in the main springs, which was pretty neat.
Have fun and dive safe,
John