Rainbow River in Dunnellon. a spring fed river with 200-ft viz and
plenty of life. It takes from 30 minutes to 2 hours to drift the river
(depewnds on if you kick or not).

You can rent a canoe at KP Hole Park, padddle up stream and drift back
or Birds Underwater (http://www.birdsunderwater.com) in Crystal River
runs dives up there.

Here is a video of the river (very slow loading):
http://www.tripleknotproductions.net/Demo.html

You can do a manatee snorkle in Crystal River with Birds too. They also
dive the main spring in Crystal River, but it's nothing special and I'm
not suggesting it.