Really depends what you want to do; the manatees are in the Crystal river area all year round but are more spread out and will not be concentrating in the springs as they do in the colder weather.
There is scuba all over Florida including the Gulf Coast just you need to know where to go more. If you have the time and a few days, I'd recommend driving say to West Palm Beach - dive Blue Heron Bridge, maybe some shark or Goliath Grouper Diving there and in Jupiter. Then drive to the upper keys and enjoy Key Largos protected reefs that probably have as much fish life as any place in the Carribean. Then drive back through Everglades National Park to the Ft. Myers area, can even beach dive in Venice and look for fossilized sharks teeth, and back to Tampa.
Let me be slightly more specific ... I did manatee snookle in 2010, so that's not a high priority.
I like diving, scuba, more ... But I don't know/not certify for cave/cavern and have never dive in fresh water ...
I love Key Largo but this is an extremely long drive to go from Tampa. I am on a business trip for 2 weeks and staying over for the weekend, I am packing my dive gears (show so much I want to dive ...)
My schedule is I can get out of Tampa Friday afternoon and than come back Sunday evening. Should I stress myself w/ the driving ? Or just go up to Crystal and look for some fresh water dive ??
That's the advise I am looking ... What do the local recommend ?