Honestly one of the dirtiest springs I've ever swam in. The water quality is fine (with low visibility due to many children stirring up the sand), but I discovered a half-dozen plastic bottles full of sand in and around the spring vent. As I took a couple out, I saw children pick them out of the trash and throw them back into the water.
Think of this place like a typical community pool. Even toward the end of the day, there were many, many more children than parents (leading me to believe many of them were dropped off to swim unattended), an entrance fee of $1, poorly run facilities, and a staff that seemed more interested in telling me to, "be careful not to let 'em steal that camera." I was told four or five times that I'd turn my back for a second it would be gone.
Needless to say, I don't really recommend this place unless you get here bright and early. The spring itself has a very interesting landscape along the bottom but isn't really explorable or interesting.
It was later in the day when I first visited Manatee Springs, so that probably already threw off the trip a little. The park was moderately busy but the swimming area wasn't very crowded. With only one entrance to the spring pool area it was a little congested getting into the water, but once afloat there was plenty of room to move around.
It's worth nothing that there's almost nowhere to stand in the pool. As far as I could tell, the water pours out of a cave between 30ft and 40ft below the surface. You can swim to the banks near the cypress trees and barely get your toes on the sand, but for the most part this is a snorkeling and floaty spring only.
Diving down to the cave caught me by surprise as I had to clear my ears three times before getting to the mouth, and even then the force of the water escaping pushed me a good five feet downstream. This free-dive isn't for the feint of heart and could be the deepest I've had to dive to find one (save maybe DeLeon). It's enough to make me want to go back in the morning when the water is clearer and the sun is higher.
Fanning Springs was the first stop on my five-spring day, so this visit was somewhat early in the morning (9am-10am). The spring itself was very clear and the park was mostly clean. I could have gone for a larger changing area, but that goes for basically any state park. In the morning the spring boil is under a lot of shade, which making snorkeling a little less fun. However, the way it's tucked into the surrounding wetlands with just a short walk down the floating dock to view the Suwannee, it's pretty magical in the morning. A few boats were parked at the floating dock, making it feel like a little microcosm of some of the larger springs.
While there aren't any caves that you can really get into with all the limestone rocks fallen everywhere, the sand boils are plentiful and fun to watch. The depression is also very deep, which allows for a lot of exploring. Many fish populate the spring pool, both in the weeds and out, and there's even a newly-constructed diving/jumping platform that was quite the hit while I was photographing the boil.
Overall, the park is very pristine, offers a variety of depths for swimming, lots of sand boils to watch at the bottom of the depression, and even a safe jumping platform. It can get pretty crowded, but it's one of the better swimming areas in the Florida Springs ecosystem.