For what it's worth-!

[Line Squirrel]

It could be argued that the cave is in fact a navigable waterway because it is, in a round about way, used for interstate commerce. Someone from another state comes to Florida, pays to get tanks filled and then pays for land access to the cave. Grating off the cave may have prevented people from out of state from coming to Florida and spending money to dive that cave. Therefore grating off the cave affects interstate commerce, and the cave can be defined as a navigable waterway.

The grate certainly impedes the natural flow of water through the aquifer and into the river which is a natural resource.