I shan't proselytize about solo diving; I trust you're knowledgeable and experienced enough to make a reasoned decision about this.
While I frequently miss the chance to dive places when I'm in FL w/o a buddy, I have found many places one can go solo. Near Gainesville, you can take a boat down the Santa Fe River. As long as you anchor in the river and don't get on private property, LOTS of interesting dives. Likewise, there are several springs dives available along the Suwanee downstream from Mayo/Luraville. Unless you are a cave diver, please resist the temptation to enter any overhead environments.
Further south, there's shallow water diving at Chassohowitzka wildlife refuge. Worth taking a boat trip through, even if you don't dive. Or you can dive -- and be paired with a buddy -- at Crystal River. Not great diving, but better then staying dry.
Just west of Weeki Wachee Springs, there's a sinkhole in the river called Hospital Hole. Drill is, rent a boat, go upstream to Hospital Hole, anchor and dive. This dive is not an entry level dive should be for very experiened OW divers, or preferably, cavern and cave divers. Don't set foot ashore; private property. Actually, if you're in a small enough boat to drift with the boat, the drift down the river from Hospital Hole to the county park boat ramp is kinda neat. Clear water in the Weeki Wachee River.
In the Ned Deloach book Guide to Underwater FL, you'll find lots of other sites listed. It's worth bopping around checking them out. I've done a lot of them; many I won't return to, but some are worth revisiting. Grab your gear and a map and go exploring. Then post here and let us know how it was.
