Blue Spring State Park. There is a raised boardwalk so you can see into the spring, which is clear so you can see to the bottom. All kinds of water life to see, and right now the manatee should be in, and lemme tell ya, that is one amazing sight!
We have quite a few springs close by (within an hour or two of WDW). If you want to rent a kayak/canoe, there are a few springs I can recommend. If you want to snorkel/swim, there are others (with overlap).