Big Fishes Cave Fresh water
Blue spring is well known for its manatees (they usually come up from St John's river during winter season). Remember that Manatees are protected animals (you should not be less than 50 feet from them).
To enter the spring, go N past the concession area, you will find steps. The cave is located at the run's N end (cave entrence at 3m deep).
See Blue Springs State park fees at http://www.floridastateparks.org/bluespring/