Good luck on your trip...
1: On the way close to I10...might swing by Panama City...St. Andrews State Park has nice tent sites, snorkle right from beach.
2: Your Manatee plans seem somewhat vague. Been there and done that so for what it's worth: I second: Crystal River (city and river name) on the West coast above Tampa offers "crystal clear" water, manatees, a couple of dive/boat shops and motels all clustered in a 2 mile area around the springs of orgin for the river which is often termed "Kings Bay". This is spring fed fresh water "cold"...you will be wanting to rent a wet suit.
Good bar, great food: Crackers...on the north side of the bay at Crystal River.